You will be fighting Vikke at the end of Stage 2 (Stadium). There will be 3 parts of this boss fight. The first one will consist of Vikke on the deck of the ship attacking you. After finishing him for the first part, he will then be split in half. DO NOT SHOOT the bottom half of his body. Instead, run away until his upper half climbs up to the top of the ship. His bottom half of his body will be attempting to kick you off the ship. Stand by the side of the ship that does not have the sides that are blocked. Make sure you are far away from his bottom half so he can successfully kick you off the ship. It may take a couple of tries for him to kick you off. Hit repetitively until you chainsaw his bottom half off the ship. Achievement will unlock right after doing so.
when you get to the phase that he splits in 2 don't attack the legs just keep at the torso, once gets on the drumset and you blast him a few time look for the legs doing a dropkick lightning kick. let them hit you on the edge of the ship and it should trigger it