This can be done in arcade or campaign modes and can be done with one or two players. Be sure to select "play co-op". I did it by myself (no player 2) in co-op arcade mode with Black Widow & Magneto.
When the match starts, initiate a super move with your first character, then raise your left arm to switch characters, quickly initiate two super moves with your second character, then raise your arm again to switch back to your first character and initiate another super move. If timed correctly, you should have completed a 4X Super Tag Combo & the achievement will pop.
- Use a launcher super move before you switch characters.
- In campaign or arcade mode, be sure to do this on the first fight because the difficulty increases with subsequent fights and your combos will be interrupted more frequently.
- Check out the YouTube link if you are having difficulty; following the process in the video, I got this on the first try. I did not create this video. Credit goes to YouTube user ObeyorDIE.
NOTE: If you do not yet have the "Power Duo" achievement, playing an arcade or campaign co-op game will also unlock that one.