In act 2 in last stand chapter :

Somewhere 2 sniper shoot you if you go at troika ( shower ) and shoot the roof . roof fall down and kill 2 grubs

And piece after of 2 sniper forward of torika one place at stage 2 three grubs come there the top of them there is one gas tube and you shoot this and kill 3 grubs .

And piece after at stage 1 of that place many grubs outside that place if you fast and throw 2 grenade in this place you kill 4 grubs .

And after this earth is cloven and emerge 4 boomer if you look accuracy you see one gas capsule and shoot this and kill 4 boomer.