Not only the final battle with Tiberius, but the entire final stage in general. You fight your way onto Tiberius' battleship riding onto a motorcycle and dodging missiles and lasers in the process. You then LAND YOUR MOTORCYCLE ONTO THE BATTLESHIP ITSELF and continue the fight on the battleship, dodging more lasers and fighting more enemies. Let's not forget that the ship tries to crush you itself. At the very end, you must destroy the Engine. After destroying the Engine, you take a Climactic Elevator Ride for your final battle with Tiberius. After defeating Tiberius, he goes One-Winged Angel on you and tries to destroy you by smashing the ground and causing you to fall to your death... But as you fly from the helicopter, Tiberius tries one final attempt to kill you and you must fight him on top of the ship's falling debris.
The chase with Leviathan at the climax of Stage 7, the Capital Tower Building. You pursue Leviathan throughout the burning building, and occasionally he'll fight you personally. He'll even throw one of the enemies at you before fighting you. Then at the end, you fight him one last time in his giant robot until you beat him down for good.