First off all I would recommend you play the entire level with the sniper bird.
The first section is against some Valkyries and the big mother machine dudes/exploding men. Hang back and let your mates wade in, using the grenade and ghost bullet.
the hard part of the level is facing the huge monsters who spew blood. You face 1, then two together and then a final one. The key here is to destroy the glowing bits round there mouth. Rather than hide behind the barricade I just stayed well back and used ghost bullet to destroy them. This makes it a lot easier. The grenade launcher also helps.
When you have to fight the two, hang back as far as possible and they quite literally cannot hit you.
For the last one, you have to destroy all the glowing bits other then the top one first then this one last. Same routine. Then at the end you need to switch to the bird with the knife for a QTE.
To make things easier, if you do get incapacitated, just immediately restart checkpoint. I did this twice and the achievement still unlocked so I know it works. Good luck.