You will want to use the second controller to give you a character to wail on for an hour (I signed in a Guest--not sure if that matters). Go to the inn and load up Arena Mode. Pick the 3rd chapter (the one on the clouds), with 5 point rounds and NO time limit. The "Unstoppable" achievement has problems unlocking if you don't actually kill the other player 5 times and simply let time run out, even if it says you won, so I wouldn't risk it here.
Press Start on both controllers to skip the intro (every time). Simply jump, straight up or toward the other player, and start moving the right stick left and right. Your character will perform a kick that sends the dummy player flying off the cloud. Press A on the dummy controller to respawn. Do it again 4 more times. You can finish an entire round in under a minute.
If you don't already have them, you will unlock "King For a Day" and "Unstoppable". If you feel like it, you can simply hit the dummy player 10 times while unarmed to work toward "Weapons Are For Babies" as well, though that will significantly slow you down. Do "King Pushy" while you're here, though.
EDIT: I used Red Riding Hood. Not sure if that makes a difference with the kicking move.