Marvel vs. Capcom 3: Fate of Two Worlds

This is a fairly easy achievement, especially in Arcade mode. Just choose the lowest difficulty setting and attack first 50 times. The AI on low doesn't attack that much (excluding the final battle, where you should still be able to attack first), so just charge in and attack. If you are nervous, use a move or an assist from afar, such as Iron Man's UniBeam.