Aliens vs. Predator

Okay, here's a much simpler, easier way which i can confirm works.
So right before the big guy comes, make sure you heal up (it doesnt matter if it's your last, you'll be fine no matter what difficulty you're on), but immeadiatly after he crashes through and does his "dramatic" entrance, turn left and jump down, towards some sort of forklift (the jumps may hurt slightly which you may wish to heal up beforehand), then run into the elevator. This is the genious part, none of the surrounding Xenomorphs can come into the elevator! Even though there's no tall walls or roofs, and even when they jump, there's an invisible wall that keeps enemies out, but lets bullets pass right through them. So make sure that if there's any surrounding combat droids, kill them first as they are priority seeing as they're ranged. Also, once the Praetorian is about half dead, or half alive, (take your pick), it will start spitting acid. Keep dodging, and remember, a) you have a pistol with infinate ammo, and b) as soon as the big guy goes down, your're already in the elevator, so level complete! I did this on the hardest difficulty, and was laughing because of how easy it is!
Hope this guide helps.

There's 2 ways of getting this achievement.

1) If you can time it right there is a ledge on the left hand side of where the Praetorian lands. Make sure he's looking at you and charges you, side step him and he'll fall into the hole.

2) Take the Pulse Rifle and Shotgun. Grenades from the Pulse Rifle are your friends, giving you time to stim or reload. Alternate fire the Shotgun at close range and each time should take about 1/5 of his health. Going this route you want to keep sprinting around.