First, use a special ability to inflict a bunk (debuff or status effect) on an enemy. Next, use a tissue (you can purchase one from the shop) on the enemy and then inflict a second bunk on it.
If you're not sure what they're affected by, you can wait to get this achievement until you have BMO in your party. Analyze.exe can reveal every bit of information you need to know about an enemy, including what bunk they're weak against..
If you do that, you'll unlock: