I'm at the nightfall bandit dungeon and I made it to the end, but I couldn't beat the last battle. Am I doing something wrong, or are my pets just not strong enough?

assuming your party is:
Slot 1 Cosmos
Slot 2 & 3
Leamhan
Aquamarine

Then the issue is the party, yes- not their levels, though.

> Remember to assign their proficiency points- Cosmos and Aquamarine only have one of their two assigned, Leamhan doesn't have either assigned.
Light should preferably be 2int or 1int/1dex, leaning toward 2int, Air should be 2Str or 1Str/1Dex, Shadow you can put into any of HP/Int/Agi (down to preference/what works for you)

> Equipment is the right type on Cosmos and Aquamarine (and okay? on Leamhan, though she'd do better with Evoker, which gives more to Int and Dex as opposed to the averaged bonus from Disciple gear)
HOWEVER, with the exception of Cosmos's Relic, all their equips are average at best (and garbage at worst, see: 'uncommon equipment is just Bad').
Aim for Epic+ rarity (rare if you can't get anything better, NEVER uncommon if you can help it), preferably within 10 levels of the pet (if you get enough reputation with Esior's Haven to unlock the hard/very hard missions, the mission satchels you get from those are a good source of lv20/25 epic and lv25 legendary gear)

Thank you so much! That really helped.