ME3 last mission [spoilers i guess]

[Sherman]

defend the missile battery. insanity. have had no trouble thus far, 95% of the game was cake compared to me2 then all of a sudden, infinitely spawning banshees+marauders and almost zero cover or concealment that works against both enemies. not only do the banshees flush me out of cover every 3 seconds but they can snipe me with their magic reave ball if I try to stay at a distance. usually moving around is fine but the damn marauders make it impossible. essentially i just dies to one or the other regardless of my tactics. also garrus and liara have become borderline retarded and either insta-die or don't do anything and then insta-die. So frustrating.

Does anyone have any insight? im a veteran mass effect player but this is killing me.