Crown Trick

The achievement itself is rather easy to do but you'll need a poison weapon and possibly some poison items. A 'battle' refers to defeating all monsters in a single room. Once you enter a room with enemies (at least two, but three or more works best), all you have to do is inflict poison a combined 5 times before before the last enemy is defeated.

How I did this was pick up a pistol that deals a stack of poison on it's last shot. This particular weapon shoots two single bullets and then fires two bullets simultaneously for it's third shot that inflicts the poison. I moved/blinked away from the enemies and fired off my first two shots into thin air and then once an enemy was in a range I hit it with my rapid fire to inflict a stack of poison. I repeated this 5 times and unlocked the achievement easily.



Alternatively, if you're having trouble getting a poison weapon the first couple of floors, maybe, just maybe you'll be lucky enough to win one on the Gacha machine on Floor 3 - City of Steam. I don't recommend this method because you won't have a lot of gold to spare to waste on spins.