This achievement, as well as High Five!, will require a second controller
This is actually very easy to unlock. When playing multiplayer, move your characters so that they're facing each other and
will appear over their heads. Press
at the same time on both controllers to high five. This will unlock:
Unfortunately, you can only high five once at a time. So find a multiplayer level that allows you to line them up quickly (Level 5 works well), do your high five, and then restart the level. Do this a total of 25 times and you'll have your CLAP! achievement.
A quick note: Doing this on the same level worked fine for me but I've heard of at least one person that had to do it on separate levels. So if the above method doesn't work just high five as you skip through the levels with
on player 2's controller.