The Geth don’t have very clearly defined Boss enemies. Instead all of their troops are good, much more so that the Reapers and Cerberus. For instance a Geth Trooper is much more effective than an Assault Trooper of Cerberus.
The Geth will swarm you in the later rounds and overwhelm you with firepower. The Geth Prime isn’t that strong, he only has one major attack and a melee, but the main attack staggers you and combined with the sheer number of troops can easily down you.
Killing a Geth Prime
The Geth Prime doesn’t have an instant kill move. A vanguard is a very effective way of taking out a Geth Prime. However, if he staggers you with his rockets you will be in a world of trouble. The standard overload, energy drain etc for the shield and warp, incinerate etc are very effective against the armor. Remember that if you combine another ability with warp you can set off a biotic explosion.
Because the Geth Prime isn’t really a nuance I usually focus down the weaker enemies and then the Geth Prime. Especially with a vanguard I charge around and kill the weaker enemies whilst damaging the Geth Prime with my area damage so that it is easier later.
With other classes it is important to make sure that you don’t get swarmed by troops whilst you take out the Geth Prime which takes a little time. Killing a Geth Prime with many enemies around with a rocket launcher is also a good way to go because if there are enough enemies you can earn a Killstreak.