I've been stuck on the battle with Desius in the forgotten tomb for a while now, and I just can't get past it. I have a full team, all level 60, 15 levels higher than Desius and the undead knights. I have all my best equipment on, but he just eats through my teams health (20%+ with each hit). Is there a story piece I'm missing? Or a specific battle strategy I should use?

Your tank pet, Kapa, barely has any agility. Agility is essential for the tank pet because the tank receives the most damage. If Fawn is your tank pet, then same thing as Kapa.
I would also advise to changing the element of either Kapa or Fawn. You have 2 defensive elements and 1 supportive element. I recommend changing one of the defensive elemental pets to an offensive element so you can actually damage Desius.
Information on Desius can be found here, just a bit of scrolling to do.
Desius's Genetic Scan:

alright, thank you! I'll do that

With Desius, did you successfully activate the four seals on the way through the tomb?

Kapa:
As an Earth pet, slot 1
His equips should all be either Assassin (non-LG type equipment) or Warden (LG type equipment) (Tidal Lord would also work)
It should be easy enough to get some lv50 legendary assassin equips for him off of the broker if nothing else.
At the moment 2 of his four equips are Int-based, and one of them is Str-based but a Dex rather than Agi type (and a disciple which are more placeholder equips)
Profs are probably okay? I've never run my own earth tank with that setup, so I'll just say if it works for you it should be fine.

Orchid:
As a Light Healer, I'd say slot 2 (but 3 works fine)
Equips are the right type (save the disciple placeholder), but again, you'll want to upgrade to better ones lv50 legendary evokers, or Pyromancer equipment.
Her proficiencies would be pest put as either 3int or 2int/1dex

Fawn:
As Air Attacker, slot 3 (but 2 also works)
Relic is the right type, but also needs Myrmidon for the other three slots. Her two assassin equips are low leveled but would be serving Kapa better since they're Str/Agi
You want to, again, get either lv50 Myrmidons for her, or Storm Caller equips.
Her profs would be best as 2Str/1Dex, since she's an air pet (I assume she was water before, given the earlier comment in this thread and her profs+equips?)