Step 1- Create a mage character.

Step 2- Find a room with the Loony Ghost enemy on floor 8 (it might work on 7 or 9, but i was able to get it on 8)

Step 3- Add 2 other companions to your run (you need to have 3 dudes in order for 10 Loony ghosts to show up)

Step 4- Get the "Armageddon" or the "Dark Pact" card before floor 8.

Step 5- Once you are in floor 8, open the floor, and if there aren't 10 or more Loony ghosts, go on the menu and click "Leave session". Rejoin the match again until there are enough enemies to do the achievement.

Step 6- Use "Armageddon" or the "Dark Pact" and damage all the enemies, in order to get them to low health, but try to not kill any.

Step 7- Wait 3 rounds and let the card to it's thing.

This might not be the best way, but it was able to pull it off.

If this guide didn't help or my writing is way to crap to understand, well then, tough ♥♥♥♥. I am as dyslexic as Charlie from IASIP.