This achievement requires you to get all four kisses from each of the four princesses in the game. The achievement also requires that you be in a multiplayer game. It does NOT require you to be online. I got this by playing with another controller, and playing only the levels required to get the princesses' kisses.
NOTE: This achievement does not have to be done in one sitting, however, if you do it in one sitting, it will only take about 15 minutes.
There are four princesses after each of the four main bosses. After the main bosses you will fight the other players to get the princess's kiss. Simply kill the other player and the kiss will be yours. The locations of these boss fights and the princesses are...
Red Dress Princess- Barbarian Boss
White Dress Princess- Cyclops' Fortress
Blue Dress Princess- Ice Castle
Note: If you are getting this achievement with an extra controller, and you are controlling the 2nd player, this level can be frustrating. There is a part where you have to walk a narrow bridge while arrows are shot at you and ice magic is shot at you. Switching between blocking, jumping, and moving with two controllers can be confusing and frustrating. Simply let the second player die, and continue to the end of the bridge by yourself. Kill the ice thieves there, and return to your 2nd players dead body. Revive him, and continue.
Orange Dress Princess- Final Battle
Note: You actually fight for the kiss after the final credits, back in the dining hall in the Home Castle.