One Big Bug Trophy
One Big Bug
Defeat the Praetorian
This is story related and cannot be missed.
Near to the end of level 5 you will come to a point where a cutscene occurs, during this cut-scene a big alien will blast out of a window, this is the Praetorian. Simply kill it and you will get the trophy.
I recommend you pick up some grenades located to the right as you can move again. The grenades take a lot of life off of the alien which gives you a big advantage, they can also knock it down which gives you a big chance to fire a lot of shots at it and kill it more easily.