Hi! I'm not super strategic about how I fight and I'm really struggling with the festival bosses. I've found Cassie Cometchaser and Mortified Mistress Molly but any general tips on how to strategize against a boss would also be good. Here are my four best pets under level 60.

Light
Shadow
Air
Fire

1. if you want the typical tank/healer/attacker strategy, make sure tank (shadow in this case) is in slot 1
(you could also go with healer-tank/attacker/attacker but it might be more difficult for you. Tank would still be slot 1 though)

2. Try to go for a minimum of lv25 legendary equips all around (but replace if a boss drops better- make sure to compare equip stats)
Illusionist for Shadow (/light tank)
Evoker for Light healer and fire attacker
Myrmidon for air attacker
overall the equip types are already pretty good, it's just level and/or rarity letting them down (eg. North Wind has a lv30 relic, but it's only a regular rare, so it's not as strong as a lv25 legendary)
(some rare equips are good, of course, but that's mainly the lost grove equips and the equips that can be bought from the town halls when your reputation is high enough with the town in question)

3. for fest bosses, air's probably the better bet, since fire is best against more enemies while fest bosses are alone

4. might want to level up the lower level pets to be closer to lv46, since zone enemies match your highest on-party level

5. fest bosses like using an unblockable stun that lasts 2 turns on the pet it hits every so often, it's very rude

6. don't be afraid of using healing items in a fight