As Mole Knight, absorb 10 hits while submerged in a single match
Set a game with 3 cpu-s on higher levels (so they are more aggressive), set the life amount to 5 (max), select the stage Plains – Grassy Clearing (Wide area, no holes) and start the match. Use the Mole Knights ability to bury himself underground, and just go to the enemy and let them hit you, the first hit you will absorb and not lose health but the second will do damage. So try to jump out before they land a second hit. Just repeat this process during a match, absorb 10 hits and you will get the achievement/feat.