Snake’s final smash consists of him jumping in the air and GET TO THE CHOPPA! The game then shifts to and over the shoulder view where you can shoot via a grenade launcher. You can only shoot 12 rounds though. To aim you just move the WiiMote and just press fire (the B button?). You can’t just sit there forever though, you are on a clock, it’s best to fire as much as you can.
If you use all your rounds, the Final Smash is over.