ULTIMATE MARVEL VS CAPCOM 3

Go into Arcade Mode, select any difficulty and select Iron Man as your character. At the start of the match, use Iron Man's Proton Cannon Hyper Combo (QCF L+M) and while it's firing, press the Y button (M) very fast, it will boost the amount of attacks in the combo.
If you do this right, achievement unlocked.

Select Galactus Mode in Arcade Mode (Highlight Arcade Mode, Hold the Back Button and LB and while holding those press A. Instead of the character select screen you should go straight into a match with you playing as Galactus.) Press QCF L+M and you'll go back, hold out your hands and shoot lasers all over the field. Gives you far over the required combo for this achievement.

Press LT at the start of the match, then spam Y.

This is all you have to do. I'm not a fighting games scientist, and this was easy to do. Get a high combo and it will unlock after the match.