Aliens vs. Predator

For this achievement you need to kill Praetorian on the Arena. It's story related and can't be missed, but for those playing harder difficulties I have a piece of advice in case you struggle this with other methods.

So, once on the Arena -- equip Battle Disc and turn on "alien vision". Climb one of those pillars on the Arena and find where Praetorian is. Get closer to that area and jump down keeping some distance from him. Ignore (or shoot out *) lesser aliens. If you will keep moving -- most of the time they will pose absolutely no threat. Focus on shooting at Praetorian with disc back-pedalling if he start walking to you. Rinse, repeat. If he charges at you -- run away ASAP, otherwise...

With some skill you can hit him twice in one shot: if you shoot and then keep disc's laser targeted at him so disc hit big alien again while returning back to you -- it's a matter of 5 or so minutes for you to kill Praetorian (on Hard) this way. Disc is the best option for (this) long fights since it wont drain your energy and you can use it endlessly.

* If you killed both little aliens stay reasonably close to Praetorian, for as soon as he start to screach -- hit him once with the disc to interrupt it. Otherwise two more aliens will appear at your "disposal".

It's a bit safe and human-ish approach but since I hate replaying boss fights I've found this method is a saviour. Hope this help.