If you want to level some pet without risking its life during this battle you can let your Crawdad die from Apocalypse and let your Beetle die because the Rat dies. The remaining pet will get all exp. It's not a lot, but personally I like to get some exp along the way.
Great failproof strat. The Unborn/Ikky strat failed for me today so this was a great backup. I'd recommend just passing and only cast Renewing Mist when it has one round left, and Wish on CD, to speed it up a bit.
This one worked during my first run thru the dungeon. The other two strategies offered for this battle both failed for me as my last pet died just after the Rat did, thereby losing the battle for me. With this strategy, my beetle died just after the Rat, but I still had the Crawdad and the third pet alive. Thanks!!
For whatever reason, this strategy did not work. By the time I got the third pet in, Autumnal Sproutling (since both the other two pets had died), it died with the Rat after the immortal round, even though I used Sons of the Root to make Autumnal Sproutling unattackable.
So, what did I use? The ever-reliable cheese combo: B/B Unborn Val'kyr and Ikky. The usual method: Curse of Doom, Haunt, bring in Ikky, Black Claw, Flock. Done. Third pet spared.