This is a story-related achievement and cannot be missed. It will unlock upon defeating Josey, the Stage 4 boss of the game.
This fight has 2 main parts.
Phase 1 - The first phase Josey we'll fly around in his UFO. Attack him with your chainsaw blaster when he drops bombs and summons stairs. There's ammo waiting at the top of each one. Dodge his lasers and when you hit him just right he'll crash allowing you to hit slash attacks.
Phase 2 - His 2nd phase is timed so don't dawdle unless going for the Critical UFO Finish achievement. Destroy the blue generators carefully as they will shock you when lit up. Once they are all down make your way to Josey and slice him in half for the finish.