Okay, so I need help. LOTS of help. Iiterally went in to this fight with 5 elixrs, 5 draughts, and all the mana, alongside powerful lvl 48 sylestis, all wearing epic quality armor, and DIED. I lost horribly even though I went in DECKED OUT. If you want links to my party just in case it's something with them, then here ya go.

1:https://www.sylestia.com/view/pets/?petid=3433385
2:https://www.sylestia.com/view/pets/?petid=3456771
3:https://www.sylestia.com/view/pets/?petid=3433294

But it is making me really salty. Because I was able to tough out multiple hordes without any elixrs or draughts. So, does anyone know what I am doing wrong? Please tell me if you know because I am fed up with it.

1. If it's a Horde battle, first tip: always go for the strongest enemy first.
(if it's in the garden of the magnates, maybe check that section in the walkthrough- it is meant to be a hard fight there)

2. Consider your pets elements: Light, Fire, Shadow- all Int/Magic based elements.
Generally people have both types of damage in their party (until they reach a level where they can start messing around with thoughts like 'I wonder how it'll go if I use three air pets on this dungeon boss')

3. Kurito has a Phys type relic, meaning he's not getting the attack boost from it due to his Shadow typing being a Magic type.

4. No tank? The general team setup is front = tank (typical elements: Water/Earth/Shadow), and a physical attacker type and a magical attacker type (light is a good type to have for a horde)
Tanks tend to be oriented towards HP and Agi, which none of your pets are.
Mysteria: 2x Attack
Equarian: 1x attack, 1x dex
Kurito: 2x attack

joining with... 5. general equipment
Higher rarity does beat higher level, but you still want to keep your equipment as high of a level as you can- you've got some level 20 pieces hanging out in there. You also need to make sure it helps the right stats. If you do Very Hard Missions in Esior's Haven you can get legendary level 25 equips- which will be better than the current lv25 epic relics and 20-30 armor. (Also healing items)
Also,

6. Decide what roles your pets have. Joining points 5 and 4, you'll want to make sure that you primarily equip stuff that boosts the relevant stats on a pet (Kurito's kind of all over the place with his equipment.)
On, say, an Air pet, you'll want to give them Myrmidon equipment primarily, because you want to run high Strength and Dexterity.
We ourself tend to run water tank/light healer(+attacker)/fire attacker.