This is easily done in co-op with real people, but for people like myself without local gaming friends (or friends who don't want to play this game) and two spare controllers, the easiest place to do it alone is this:
In the very first level, the second fight will demonstrate how to deal with enemies that are blocking your attacks. The first and only enemy that appears in the first part of this fight will simply block and not move.
If you have your main account and two dummies/spare controllers in the game at this point, you can take all the time in the world to walk them over to this enemy. Then you just need to hit the X button on all three controllers at once. Even though the attack is not effective against the blocking enemy, it counts for the achievement. All three accounts will unlock it at the same time.
[[The fact that it unlocks for all three people might cause the achieve-pop-notification to not pop up for your main account, as it did with me, but just check your achievement list. If it pops for anyone, it should be unlocked for you as well. Of course, it's always sad to not have the unlock notification / the 'pop'.]]