Apply 99 or more Poison on a single Enemy.
Catalysts, preferably upgraded and having multiple copies, are essential here because you’ll never stack 99 Poison through small increments. After that’s done you can do this on floor 2 boss. High HP encounters on floor 3, and the floor 3 boss.
Noxious Fumes is good enough to start accumulating poison though Bouncing Flask is much better, then you just need to play the Catalysts when the time is right.