Watchmen The End is Nigh

This achievement is much easier than it sounds. Just play through the game normally. When you hit Act VI which is the last level, you'll be chasing the final boss through the level. At the end of the level he will jump on a set of suspended beams and start shooting at you with his flamethrower. In this part of the fight several minor enemies keep flooding onto the stage because you're supposed to throw them into him to get him off of the beams. Instead of throwing them just go ahead and fight them. There is an infinite number of them and they are much weaker than most enemies so just keep fighting them until the achievement unlocks.

REMEMBER: Stay to the side of the stage. If you stick around too much in the center you'll get hit by the flamethrower and take a LOT of damage.

Once the achievement unlocks you can progress the fight by throwing some of the weak baddies at the boss.