I go with the undead, ooze, misc pet (with a magic attack) strat. Sometimes I don't even need the third pet.
Any undead will work so long as it has the Wild Magic ability (Fetish Shaman
, Sen'jin Fetish
, Voodoo Figurine
). Attack Chi-Chi normal in any fashion, but on your undead pet's final turn (when it rises from the dead), use Wild Magic.
Once my undead pet is dead, swap in my ooze and although Chi-Chi will do his avoidance move right away, I just keep Corrosion and Acidic Goo up as much as possible. (Note: I use a P/P Jade Oozeling
; I'm not sure how any other ooze of any other breed will work.)
On good days when RNG is in my favor, I don't even need the third pet. On days where RNG is not so kind, I have a back up pet with a 100% hit magic ability waiting to finish off Chi-Chi.