The dumb Praetorian in the Research lab...Impossible?

This is exactly where i was stuck in the course of writing my Nightmare Marine Guide.

Use shotgun/pulse rifle instead of sniper/pulse rifle. Knock her over with grenades then double-barrel her until she's dead. Remain up on the stairs where you begin, slowly fall back around the corner, deal with any xenos that come. When praet. appears grenade her feet, run around her down to where SHE began (give her another grenade when you're clear of the blast radius). There should be more grenades and shotgun ammo down here. Just keep grenading/double-barrelling her, whilst taking care of any stragglers. So long as you keep some distance between you and her you should be ok.

Sounds like a lot but it's a lot simpler in practice. Once you defeat her, stock up on pulse rifle/sniper ammo, head down elevator to complete the level. Keep your eyes peeled over the next couple of weeks for my Nightmare Marine FAQ (have to polish off my Nightmare Predator Guide before i post it up) to find out how to do the final level quick and easy.

There is a trick just run two the left over the rail just as u see him then get on the elevator so the minions wont get you. this can make any difficulty easy but watch out for her spit attack

Like renog1996 said, there is a glitch to where if you run to the elevator, the minions nor the Praetorian will not be able to attack you. However the Praetorian will still spit acid at you.