This can be done both on the ground and in the air. He simply fires from his arm at a fairly rapid pace which can keep many enemies at bay for a fair amount of time. Using Megaman, I personally just liked spamming R1 presses as this is his mega buster move. Megaman also has a move with the same input that throws down a mine which pops up the enemies. This is an effective projectile that pushes them back and is fairly powerful. Quarter Circle forward+ Triangle using War Machine gives you the shoulder cannon. Moves you can use to cheese your way through the first few fights leading up to the boss are as follows. That second part may vary depending on your play style, however I preferred doing big damage to the boss and defeating him with some meter rather than being able to endure his attacks. First and foremost, you'll want to go to the game settings under options and lower the difficulty (although in my opinion it doesn't help much) and raise the damage level to maximum. My preferred team for unlocking this was Mega Man and War Machine as they are easy characters to cheese projectiles with.

Marvel vs capcom origins psn