You'll fight the Hive Mind at the end of Chapter 12.
You'll notice like the previous bosses, the yellow boils are the weak point of this one. There are two sets of five, one in around it's mouth, and a second inside it's chest cavity. A fully upgraded (at least for damage) Plasma Cutter is enough for this fight. If you have max health and a Level 5 or Elite Suit on with a nice number of health packs, you'll have no trouble with it.
You'll start with the mouth area. The Hive Mind will "scream" at you and leave it's mouth up close for you to quickly fire at one of the boils. Destroy it and the creature will rear back and slam at you with it's two tentacles and possibly an electric attack of some sort. Dodge them, though if you get knocked down you shouldn't take much damage with max def and any damage can be quickly healed.
Repeat this twice to lower the number of boils down to 2. At this point, the Hive Mind will reach out and grab Issac and lift him up into the air. Similar to the previous battles where you were grabbed by a tentacle, you'll have to aim at the boils on it's face to stop it from killing you. The angle is more confusing than before, and you're being constantly moved about, adding to the challenge. You'll have at least 3-4 chances as you're moved closer to destroy the remaining two boils before you're killed.
With all the boils on it's mouth destroyed, the Hive Mind will drop you back to the ground and rise up, exposing it's chest cavity and the last five boils. Quickly fire into the chest cavity, they are close enough together that you'll probably damage more than one of them before you actually destroy one, making them easier to destroy later.
When you destroy a boil, the chest cavity will close up once again and the Hive Mind will slam down with it's two tentacles again. To avoid these, simply strafe from side to side of the battlefield with the run button down. It will attack with the tentacle on your left first, then the right, then left, then right, then open the chest cavity again. Repeat this 4 more times and the boss will fall and you'll get the achievement. Run up into the shuttle and you'll also unlock the achievement for completing the chapter and the game.