DO NOT pass the undead phase. Use Stampede there, giving you a turn less of stampede on the 2nd pet. Black Lamb was able to solo the 1st 2 enemy pets by Stampeding turn 5, 6 and 7 then using Comeback twice.
Thank you for your feedback. I tried this out, and this will likely depend on what moves Corrupted Slime uses, because they aren't always in the same order. Sometimes I don't survive beyond two stampedes, and sometimes it's as you described. When I make a strat, I try to use what will work the majority of the time, and not depend on the enemy pet using moves in a certain order when the enemy pet's moves are random.
Used Rat 2, 1, 1, for comeback and stampede Leonard didn't use sweep so finished him with rat and used comeback til dead on 2nd pet, Fire beetle roasted corrupted slime and got 3rd pet to half health. used scooter the snail with absorb shell shield and headbutt. only needed headbutt to finish tentacle