Foom's Doom Achievement
Defeat Fin Fang Foom.Story-related; cannot be missed.
This fight will take place at the end of Chapter 13, and is a relatively simple fight since there are no other enemies except Fin Fang Foom.
He will do a number of different attacks, all of which are telegraphed with a red exclamation and are easily avoided. You can shoot at him now but you'll barely do any noticeable damage. After enough of his attacks he'll shoot a heat beam into the sky (shown by a cinematic the first time) and make ice chunks fall to the ground. Avoid them, and after around five seconds of dodging look toward Foom for a prompt to interrupt his attack and stagger him. He'll fall to the ground, and at this point unleash everything you have at him to do decent damage until he recovers. Repeat this process probably three more times to finish him off.