I have a Light, Shadow and Water pet on my team and all are level 60. I still can't beat him! I need help.

Okeeee... Here we go. This is just what I've heard, I'm currently on him too. So, your light and shadow pets need illusionist items equipped, or things that specifically boost their two recommended stats besides health (viewable on proficiencies page). Same with water, only they need assassin stuff. Make sure their equipment is the highest level you have. Also buy tons of elixirs and draughts for healing and revival that don't say "this item cannot be used during a battle." I hope this helps and please don't get annoyed at me if you already knew all this...

I kinda knew about the illusions for the light but nothing else you mentioned i knew. Thanks :)

Okay, glad I could help. One more thing, you can use the elixirs during battle by clicking the red arrow next to the brown sack under your opponents' stats. Wasn't sure if you knew, cause I sure didn't before coming to the forums!

try getting an Air or Fire pet, cannons are real helpful when whooping an opponent's tail, especially when the team being used against the opponent is overleveled, and you'll want your Shadow pet in slot 1 since it's considered a tank. I beat King Desius with the following party:
Slot 1:
Name: Blacky
Species: Lupora
Element: Shadow
Level: 60
Gear equipped: exotic Reaver gear I bought off the broker
Random unnecessary fact: she has Jeweled Horns with a hidden carry of Crystal Ice Crown
Role in battle: tank
Slot #: 1
Slot 2:
Name: Lucy
Species: Ryori
Element: Fire
Level: 62
Gear equipped: exotic Pyromancer gear, partially from another player and partially from the broker
Random unnecessary fact: she is a 2-vis 2-carry with 1 hidden carry, her traits are (AC) Shock Runes, and the C shows that she has Double Stripes underneath her Shock Runes as a carry, (NA) Nebula, the N shows that the trait is there but carried, (NA) Ethereal Armor, and (FF) Shadow Wings
Role in battle: cannon
Slot #: 3 for other people, 2 for me(yeah I'm an oddball, so what?)
Slot 3:
Name: Crystal
Species: Zolnixi
Element: Light
Level: 63
Gear equipped: exotic Lightbringer gear(can't remember where I got it from though, I don't have access to the Lost Grove yet, in fact, I'm on the last location in Umbra Forest, so I'm nowhere near the Lost Grove, I don't think)
Random unnecessary fact: he was my second pet and has Fallow Deer and Chow Tail with hidden carries of Royalty and Feather Dust
Role in battle: healer
Slot #: 2 for other people, 3 for me(like I said, I'm an oddball)
try to get Lost Grove exotics if you can afford them

you need to do the seals. Once you do those hes really easy to defeat

How to do it, if your ok with spoilers
https://www.sylestia.com/forums/?thread=44033

No offense but the majority of that is unnecessary information...and really, level 60? seems like a bit of overkill.... I beat him around lv 40 pretty easily

well, my pets were only level 60, 62, and 63 because I overleveled them in the Howling Pass during the Winter Fest. But it's good to have strong pets anyway.
Edit: it's good to have strong pets in my opinion, anyway. Others may be fine with level 40 pets, level 50 pets, etc.