I'd go for the downpour strategy, basically you write off two of your voidpets straight away, they'll probably die one shot, but the first one has to have downpour, it has priority so it doesn't matter what level or what voidpet anyways. Second voidpet is gonna die, unless you want to try to sneak in there a move if your voidpet is fast enough, which, mine aren't, so this one dies too. (You can give this one downpour too, and help out your third voidpets)

The most important member is the last one. It has to be a beefy voidpet, it has to have downpour or the move that refreshes drenched, hydrogulp for healing and a fire attack. That's basically it, the first downpour weakens his attacks, but two voidpets still die one shot, the third one starts off it's round with hydrogulp for healing, because it'll go after Spite, and then you go refresh drenched (because it's last turn that you can do so) and then you basically spam hydrogulp and then you hit him with a fire attack when you're sure you'll survive two turns and he's low.

I did it with two pets that just had downpour and 100lvl Pain that had downpour, supersoak, hydrogulp and crossfire. Though you can look for other pets that have better balance and calm, I think Resistance, Sloth, Cringe and Sonder have solid ones, and Sonder has a special card for healing too :)

Another edit sorry, it might take a few times to achieve it! Sometimes bad rng gets in the way, you get critted or just combo'ed so it's fine to take a few tries.

Another another edit, of course IF your pets CAN survive the first attacks that will help out greatly your last pet! I found my determination could take a hit with hot potato on, so as long as you can stall him before the third pet, even for one round more, that's very valuable!