Act 2 is called the "Ice Forest" and has four sections: 2-1, 2-2, 2-3, and 2-4.
The boss at the end of 2-4 is called Hod. Break the control box next to the barrier to disable it, and then you will have 5 pillars on which Hod will keep jumping between while shooting at you.
Destroy each of the 5 pillars he stands on with gunfire, and when they are all destroyed, he will fall down a level. Go back in the elevator and down to the next floor and repeat the same process 2 more times until he falls to the bottom floor.
When you get to the bottom floor with him, you can attack him properly so rush in and attack him with melee attacks. Using finishers also does massive damage to him.