Here is the full list of all 22 Mayhem Heroes achievements.
Hit hard the gong located at Rising Sun level
Win a versus round with "most kills" victory condition
Win a versus round with "last man standing" victory condition
Knock down 5 opponents in the same round
Knock down 15 opponents in the same round
Get knocked down 10 times in the same round
knock down 5 opponents without being knocked down
Die knocked out without hitting anyone
Land 3 rage super attacks in the same round
Kill an opponent with a super rage attack
Land more than 50 attacks in the same round
Kill a human opponent in any game mode
Perform a teleport
Get hit by a meteor
Win a survival round
Win a tournament
Move up more than 3 positions in a tournament
Collect 50 power ups
Kill an opponent and steal their power up
Deal over 500 attacks
Calm down an opponent by knocking him out
Hit a friend in survival mode