I suppose I might as well finally acknowledge that the world record for this level fell from 0:06 to 0:05 way back in 2010, and the strategy below is even less useful for obtaining a time of 0:05 than it was for getting 0:06.
For my money, the canonical guide to getting 0:05 Defection A is this.
Original strategy, 2001
So you're a Perfect Dark veteran. You've seen it all, you've done it all. All the golds, all the challenges, all the solo missions, all the co-ops and all the cheats. Game over, right? Wrong. Why not try going for speed times? How fast can you do Defection Agent? Maybe your record is 10:04, because you only ever did the level once and never went back. Or maybe you decided to knock down your best time a bit, and you got 0:43. Or 0:39. Or even, if you were a world-record holder in mid-2001, 0:33. That's the limit, right? You feel you just can't go any faster. It would be physically impossible. Right?
Wrong. Try six seconds.
Some people don't believe what is written here. They say it's impossible. But we are the Perfect Dark Elite, and we laugh at the impossible, and with my help, you can too. Only the pure of heart will pass. Ready?
The secret to the six-second glitch is an almost unnoticeable flaw in the structure of the Lucerne Tower's roof. By utilising this flaw it is possible to sidestep through what appears to be a wall, out of the side of the skyscraper, and plummet past the scraper, past floors of offices, right to the ground floor lobby, all in a matter of seconds. To get 0:06 you'll need to actually land inside the elevator at the bottom of the building. However this is easier said than done, and there's a nice selection of other places you will land before you actually get the glitch to work in the way you want it to. Don't worry about this. Nobody gets it right first time.
The glitch is very hard to get right the first few times, but believe me, it does get easier with practise. To start with I'll send you on a little training course - you're not going for a world record (WR) time here, just perfecting the technique. Start up a normal run on Defection Agent, and skip the cinema. Turn ninety degrees to your left from your starting point and drop off the helipad. You can kill the guard patrolling around here and the one inside, if you like, but they don't actually pose too much of a threat. Go to the first door that leads into the building and open it. Go in, turn around, shut it completely, then open it again, this time outwards. Time for some speed. Hot-foot it up the ramps back to your starting point on the helipad. Go to the edge where you dropped off before, and look down to find the door you just opened. Back up a bit and take a running jump (well, not really jump, but you know what I mean) off the helipad and onto the top of the door.
I bet you never had this idea before, huh? Now it gets interesting. Balance on the door. The platform is fairly large so you won't have too much trouble staying on top. Also, the door won't close while you're standing on it. Turn left ninety degrees to face the light above the door.
The panel you are looking at is the flaw in the skyscraper. You can shoot it as much as you like and knock out the light while you're at it. However, you can also walk through it. Crouch once. (Note: the fence above it is real, that's why you have to duck under it.) Not twice, once. Now slowly edge forwards along the door, being VERY careful not to fall off the door at this point. Eventually you'll walk through the panel itself, and you'll find yourself standing roughly on top of the where the door hinge is, looking around in near-total blackness. Freaky, huh? NOTE: if you uncrouch now, you'll pop up on the skyscraper roof, so you'll need to do all of the above again.
Take the time to have a leisurely look around now. If you're uncertain, hold R to make sure you don't accidentally move and fall off the door. Interesting tourist attractions around here include: the traffic which zips past; the mysterious chair (look down and left); the big silver X on the other side of the building; strange glitchy panels; and the pipes' interiors (right). You can also shoot in any direction you like at this point and the bullet will impact right in front of you. Impact on what? Heaven knows.
Day trip over, on with business. You've exploited the flaw but you're yet to activate the glitch, as you may have noticed from your lack of falling-ness. To actually begin falling you need to run right, through another panel, out of the side of the building. But it's not as simple as that, there's only one specific direction you can run in to do that. And it's not easy to find.
You know the two rotating vent shafts you can see giving out light on the roof? These pipes are continued lower down in the building. It is the inside walls of these pipes that you can see to your right from your current vantage point. To do the glitch, you are aiming to strafe forward and right between these pipes, hopefully aiming to just brush the right edge of the left-hand pipe. Got that?
Turn to face directly forwards once again, and then turn diagonally so that when you start running forwards and right in a moment's time, you will just brush the inside right wall of the pipe on the left. In this position you can see all of the left-hand pipe, with a gap of about one inch of blackness between the edge of the pipe and the edge of your screen. You will be able to see none of the right-hand pipe.
Now hold forward and right-strafe (whichever buttons those happen to be on your current controller layout), until things finish happening.
There are about five things that could have happened. The worst one is that you fell off the door and landed in front of it - this means you missed the panel in the right-hand wall that you were supposed to go through, or possibly that you fell off the door by accident. Maybe you crouched twice by accident, that'll have the same effect.
Alternatively, you may have seen a brief flash of black and red and landed by the pipes and the internal comms hub - by comparison, this is very good. This means the glitch worked, just not in the way you were hoping. Give yourself a point. A third possibility is that you landed in one of the pipes and fell a short distance into the secret room with the cheese. What do you mean, you never knew about it? This is equally good, give yourself a point. Both of these two possibilities mean you hit the right patch of wall but you weren't going in quite the right direction, and collided with the pipe instead. On your next attempt, try a slightly different direction - hopefully by now you'll trust me that the glitch does work.
The fourth possiblity is that you saw a fleeting flash of a lot of glass, then plummeted to your doom and died. You found the right place to run through, you ran through it and you were outside the skyscraper. If you had the presence of mind to look left, you might even have seen the skyscraper's upper floors zooming past you. This is very good indeed - give yourself ten points. Finally, if you both made it outside the tower AND landed somewhere on the ground floor, you did everything right, and you even thought to run back in under the skyscraper afterwards. Congratulations. Give yourself a million points.
The chances that the glitch worked at all on your first attempt are very slim. This is perfectly normal. If you can, close and open the door so it stays open until you land on it from above, then go back up to the helipad and try again. Or just pause and restart the level (you'll have to do this if you ended up trapped with the cheese). Try a slightly different direction each time and, most importantly, remember which one works when it finally does, or you'll never be able to do it again! Practise, practise, practise until you can get it to work frequently enough to satisfy yourself. One success in four attempts is good enough for me.
Here is what to expect during the fall. Most of the world to your right will turn black, while to your left the next four floors of the skyscraper will shoot past, including a prominent grey platform encircling the tower at the bottom level. After the last office floor vanishes the all you'll be able to see is blackness and, if you look in the right direction, the moon. Looking downwards you'll only see the ground floor a few seconds before you hit it. It's actually all rather restful. It's certainly very enjoyable the first couple of times you actually pull it off, because it's absolute proof that you don't suck.
Now, how to apply this astounding technique to a real world situation? There are two options available to you. The first is to do it like you did during the practise runs - drop down, open the door, run up to the helipad and give it a go. However the time taken to do this negates the time saved by doing the glitch. There is a more advanced technique. To explain this, stay on the helipad and watch the guard down below patrol around for a while. (Shoot out the landing light if it's bothering you.) See? He goes in the door, then comes out again. Start a new level and watch the opening cinema until Carrington finishes saying "...the key is useless." Now go to the edge of the helipad again. Voila! The door has been opened for you by the patrolling guard! This, used with skill, should mean you can activate the glitch a good ten seconds earlier than usual. Without this trick, your best hope is roughly 0:22. With it, you can get 0:13 or even better.
Once you're through the wall, landing on the ground floor can be a hit-and-miss affair, at least the first few times, but it's actually very easy indeed. While you're falling you are of course outside the skyscraper and if you stay there, you'll miss the ground and die. However once you pass the lowest level of offices, you're able to turn left and run straight into the empty hollow of the skyscraper's interior. Once here you'll land right in the lobby with ease. (Fortunately, the drop won't kill you.) IMPORTANT NOTE: after you are through the wall, you are still crouched and for this reason you will continue to move slowly. You should uncrouch as soon as you are through the wall.
The worst problem I find is orienting yourself after landing. You'll have to look around a bit probably to find out where you are, and then make a dash for the elevator. Depending on where you land this can take between no time at all and ten whole seconds. After a few successful jumps you'll learn exactly how the ground floor is positioned relative to you... You'll then be able to work out how to land wherever you like, facing in the right direction. However, ideally, and for maximum time-saving, you need to land right in the top of the elevator. Your best possible time without doing landing in the lift is about 0:13, whereas if you pull it off you can get 0:09.
Landing directly in the elevator or the immediate surroundings takes more than a bit of judgement, skill and failing that, luck. Instead of waiting until you pass the last office floor, the best way to hit the elevator directly is to get caught on the grey platform that protrudes outwards from the last office floor. You've probably already seen it flash past a few times already - you may even have landed on it. Hitting it isn't much of a problem compared to getting the glitch to work in the first place. Once on the platform, turn left to face the offices. Some words of warning - you CAN fall off it if you are careless while running around on it, so take care.
There are two ways you can jump off the platform. The first is to just back up and carry on falling from when you left off. The second is to go along the platform and find the grey-green panel right on the corner, and slowly walk forwards into it. You'll fall through and plunge towards ground level. Either is good, but the inner one is considered faster while you're on a speed run. The lift is very difficult to locate from above while flying blind. Generally, here's what I'd do: Facing the green panel, run forwards for two seconds, then strafe right for one second. This plonks you roughly in area of the lift but I honestly can't guarantee it'll work every time. The only real method to finding the lift is to remember carefully exactly what moves you made after dropping off the platform, and where you landed, and to adjust your movements accordingly. NOTE: if you do manage to land right on top of the lift, you won't have to activate it to finish the level. The level ends automatically.
Having mastered the technique of finding the lift, your best possible time decreases from to 0:09. (0:14 is a very respectable time - in fact, getting the glitch to work at all on this level will be worth at least a couple of points on the PD Elite rankings.) Now, there are still a few more seconds you can save. Landing on the grey platform slows you down from falling at top speed to zero instantly. After that, you start falling from zero speed again. This brief stop-off wastes a lot of time. The masters of this glitch don't actually use the grey platform, but drop past it and run in the right direction to make it work. Although this works if you get it right, you need to be VERY careful. Getting from the glitch point on the top floor to the correct position requires a lot of running in mid-air. If you hit the grey platform you have as much time as you like to line yourself up correctly. However, if you skip it you may find yourself unable to run far enough, and landing by the dataDyne symbol in the main lobby instead, because you fell short. To combat this you need to uncrouch as soon as you possibly can once you've done the glitch - in fact, ideally, you should do it just as you pass the red pipe. Then run in midair so you drop past the grey platform just next to the green panel that you ran through before. If you manage this, you're on course for the lift, and your best possible time is no longer 0:09 but 0:07.
So the 0:07 technique is - wait until "key is useless", jump on door, crouch, crawl, jump, skip grey platform, hit lift from above. But it's not exactly 0:06, not exactly the world record, is it?
Before you can get 0:06, you'll need one last piece of information. This very advanced technique involves waiting in the opening cinema until the very last moment before the jumpship appears on screen. (You should hear it go "whoosh" about a second into the level.) Now start, go to the edge of the helipad, and wait until the guard has stopped to open the door inwards. Jump now, and with some incredibly precise timing, you can land on the guard's head, just a fraction of a second after he has started to walk forwards. React quickly and crouch once - this will stop him moving out from under you. Now line up carefully, hopefully with your head in the light panel like before, and try to do the trick. If successful you'll go right, over the top of the inwards-opened door, through the panel, and succeed. Still, this won't save you much time - you waited too long for the guard, right? Restart the level, and wait until just after the "whoosh" has finished, and the jumpship is small in the distance, then hit start and forget looking down - just jump and crouch in mid-air. If you're a real gaming genius, like a number of people on the PD Elite rankings, you should bounce off the guard's head and through the panel in 0:03 or less. Now plummet to the lift at your leisure, and if you're a true master of the art, you may get a drop-dead perfect run, and make it in 0:06, joining only a handful other people in the world in jointly holding the WR.
There a lot of people who wonder if 0:05 might be possible. Sadly, it isn't. I quote myself from December of 2001: "Given that 0:06.98 has been achieved using the door, and the world record is 0:06.54, it would seem strange that you only save 0.44 seconds by jumping off the guard's head - but this is actually totally correct, all things considered. Suppose we wanted to get 0:05.99. Suppose also that no rounding occurred. That involves saving 0:00.55 from the current record. There's a limit to how fast you can fall. Once you're out of the wall, everybody, regardless of ability, falls and hits the elevator at exactly the same speed. This phase takes around 3.9-4.3 seconds, I'm not sure exactly as I can't see decimals on the video. As a conservative estimate, let's call it 3.5 and be generous. This leaves 3.04 seconds in which you are not falling. In other words, there are 3.04 seconds in which the speed you go at is determined wholly by how skillful you are. Let's also take into account the 0.5-0.8 seconds wasted waiting while you fall from the helipad to the guard's head. This leaves 2.64 seconds (again, a conservative estimate). 2.64 seconds of incredibly frantic, high-speed play, during which you jump from the helipad, crouch, slide right and begin to fall. From this, at least 0.55 seconds are to be saved. Impossible. Even in slow-mo. May I go on record as saying that 0:05 Defection Agent will never be achieved." And I think that covers it.
Congratulations go to ParagonX9 for his superb job of reverse-engineering the game to figure out the glitch in the first place. See how the amazing story unfolded here.