Yeah. The title says it all. I have 2 zolnixis, both around level 37, one shadow element, one light element, going up against a few level 25 undead knights, and both keep dying. I would get a 3rd slot, but I don't have enough. Any tips?

I would defiantly keep working at the third slot, also be sure your equipment matches you elements

Okay! Thank you :)

If it's a hoard/swarm/whatever of enemies, I'd also suggest Fire element, which hits multiple opponents.

Looking at the two pets you're apparently using (going by the elements you mentioned), Kurito and Mysteria...
Oh boy.
The general levels of the equips are acceptable, being between 15-35, though you do want to aim for higher rarity stuff as much as you can, rather than uncommon or rare.

Both of your pets are Magic damage oriented, however, and BOTH have Physical-damage relics. Basically, they have no relic boost to their attacks at all. If the two of them had correct damage alignment- exactly matching what they have, but just with the correct damage type- Kurito would go from 111 to 180 (+69), and Mysteria would go from 135 to 240 (+105) damage.
You need to look at what type of equipment you're giving your pets; Int pets are oriented towards Evoker (+int, +dex) and Illusionist (+int, +agi) gear. Assassin (+str, +agi) and Myrmidon (+str, +dex) gear is useless to them because it boosts the wrong attack stat.

Anyhow, best bet for raising money is missions (and at the moment, also doing fest stuff. While it's rare, even wooden tools can sometimes get 50k gold from the Beach Treasure Seeking.)