So I have tried a few times and failed each time. Can anyone help? And does activating the seals help at all?

I'm kinda lost, battles aren't my strong suit.

You need to figure out how to activate all four seals if you want any chance of beating him.

I don't know if the seals work, 'cause i never activated any of them, but i can give you a few pointers for your pets

First of all, you should put at least 1 proficiency point into each of your pets, because they only have a little more than 1k health, and Desius has way more, keeps beating you, so you need to survive. you also may want to switch your fire pet into wind, because you want to focus all your attention on King Desius first. Put shadow in the first party slot, light in the second, and fire/wind in the last

Other than that, you have A LOT of problems with your equipment. First, all your pets specialize in magical attack damage, and you gave all of them Myrmidon relics, which boosted physical attack damage, and did not help at all. Also, Myrmidon does not help any of your pets. Your pets need Intel, Agility, and Dext. if you switch one of them to wind, then yes, you can give them myrmidon. But the other ones either need Illusionist or arcanist. Assassin is fine too. Why? Those are the stats that your pets are focused on. You can see the needed stats in the proficiency page underneath the adding points place. And all equipment gives a description on what it boosts and what it doesn't boost. Do not use disciple. It does you absolutely no good.

I'm not exactly sure if theres a strategy for this fight, because i basically just kept hitting him and then defeating all of them. Hope this helps!

I'll try it out! Thanks for the tips!

You're welcome! Good luck!