Story Achievement, unmissable. This boss has 4 phases.
Phase 1: During this phase just shoot him with the shotgun when he gets close. He will go down rather quickly, disappear, and then cut through the floor beneath you.
Phase 2: In this phase you must dodge his attacks and then counterattack, much like any other boss. Learn his attacks and exploit their weaknesses. After wearing him down you will be prompted to perform a grab attack on him.
Phase 3: Phase 3 plays nearly identically to phase 2, except he has a new attack and he has a lower recovery time between attacks. Once again avoid his attacks, hit him while he's recovering, and you will once again perform a grab attack, this time removing one of his chainsaw arms.
Phase 4: During this phase you use his own chainsaw against him. You can't roll with the chainsaw, but he really only has one attack now, and it is very short ranged and leaves a huge opening for a counterattack. Beat him down with the chainsaw until you are prompted to grab him, and the fight will be over.