This is the final boss achievement.
First part involves in a lot of jumping, know the patterns and throw grenades where the skulls will appear. You can see this in the following video:
Sorry, this Game Clip has been removed
For the second part i lost footage, but it is easier, as you need to hit it only two times. The ideia is to avoid being hit by his arms while jumping, and then killing all enemies on the screen, just unload you gun at him and then repeat one more time.