If the whole point of this strat is to spam "Dreadful Breath," why wouldn't you pop "Cleansing Rain" on the first round? i mean, simply sailing through the fight on bare minimum would take longer, right? This attack specifically mentions in its tooltip that it does more damage under "Cleansing Rain."
I used Scourage Whelpling, x, x, 2 , Ghostly Skull, 1,1,2, Scraps, 1, 1, 2.
Put Whelpling in slot one, then skull then scraps
With Whelpling first move, pass. It will be changed out to scraps.
Switch to Skull, use shadow slash until Clubber is under 600 hp then use ghostly bite. This should kill Clubber.
Squirt comes in. Pass, shadow slash x 2, then unholy ascension
Bring back whelpling, dreadful breath until you die
With scraps use claw until squirt dies.
Use superbark. IF Squeezer uses bubble, you use consume corpse to refill your life.
First 2 claws take down bubble, third one should kill him.
I tried this strat twice and the second time I got a critical hit and this strat still worked. Good luck.