You've made it to the final stage of the Cossack fortress! It's time to confront the mad scientist head on.
From the roof, you'll make your way down into the upper sections of the fortress.
There's nothing too noteworthy about the level itself, as there's nothing here you haven't already encountered in some shape or form before. But early in the level, you can use a Drill Bomb to take an alternate path through the stage. Both paths will eventually converge, and you'll reach Dr. Cossack himself.
Cossack fights you in a crane machine with a massive claw on the bottom. You'll want to unload on him with the Dust Crusher, using the ledge at the right of the room when possible. The main secret to winning here is to avoid the claw and stay on the move.
Once he's defeated, you'll discover his true motive for fighting you, and discover the true evil mastermind of this plan. The fight isn't over just yet!