Story related so can't be missed.
SOME TIPS (with possible spoilers):
-Use the cover in the middle of the middle platform against the final boss.
-The eyes and middle of the chest are the weak points.
-Save your heavy weapon ammo for the Harbringer(s) and final boss.
-Move around to collect ammo when you get low, and try to stay under cover as much as possible.
I personally just used my sniper (I upgraded 5 times and had the Geth model) on a majority of the enemies because it was a one shot, one kill deal. I used the AR only when I ran out of sniper ammo. The best heavy weapon to use imo is the beam rifle (the Collector weapon) because there's no down time for reloading.