Ok, so I have been trying to defeat desius for like a month and I've leveled up all the way to 49 and I still cannot defeat desius and he is lvl 45, I do not not know what is going on but I so need help please! I have changed my relic, armor, and accessories many times but still no! I have backtracked hoping to level up but it is just not possible when it comes to defeating desius

This doesn't belong in Arena discussion (the arena is a specific area in the third region's city), it belongs in either battle discussion or mission and quest discussion

but anyway
1. did you successfully activate the seals on the way through the tomb?

2. since they're all lv48 or 49, you might as well grind your pets up the small remainder to lv50 and get them lv50 epic or legendary equips?
But on their current gear, the main issue is the two uncommon accessories on Lavender Desires and Ash and the non-low-rarity Ravager?? That is on Midnight the light healer?? for some reason??

3. you might want to do 1str/1dex or 2Str profs on LD and 2Int on Midnight? It'd get you a higher damage output from them both, and higher output from a light healer means better healing for the party.

1. I have looked at amarok's sylestia walkthrough and I have chosen Ash and Midnight as my pets for the seals but for some reason they don't glow up or is that even supposed to happen?

2. Unfortunately, garden of the magnates will not work to level me up cuz I only get 1exp then. I can try using the dungeons I have unlocked but I do not have any more accessories for my party but if you can, please tell me which ones to use.

3. I don't really know what 1str/1dex or 2Str profs on LD and 2Int means?

1. the seals require a stat that is over 200 without taking equips into account- so in order, 200+ agi, then str, then dex, then int.
Of your three party pets, only Ash has any stat high enough to activate one of the seals- in her case, the Agility seal (her equips make up 122 points of her 341 Agi stat, so she scrapes by with an agi of 219 when equips are discounted). Lavender Desires falls one point short of being able to activate the Dex seal (equips make up 103 of her 302 dex stat, meaning equipless she lands at 199 dex)
One trick when doing the seals is to swap in Mission pets, since they tend to have all their proficiency points put into the one stat that they're a mission pet for.

2. Missions. You can send them on missions. Or use exp orbs
The pet who needs the most exp to reach lv50 is Lavender Desires (henceforth shortened to LD because that's a long name), needing 74394, which is just a little more exp than you get from a lv51 exp orb, so you could get them all to lv50+ with one lv50 exp orb per pet + 1... lv10? exp orb extra for LD

If you prefer to save the gold you might need to buy exp orbs, then the missions option.
Ash (35,692 to lv50) would only need one Exceptional completion of a lv20 Hard esior mission (42500 exp) or Satisfactory completion of a lv25 Very Hard esior mission (36,750 exp)
Midnight (62,855 to lv50) would need Exceptional completion of a lv25 Very Hard esior mission (73,500 exp) or Exceptional completion of a lv45 Hard Saerielle mission (92500 exp)
LD (74,394 to lv50), Exceptional completion of a lv45 Hard Saerielle mission (92500 exp) if you wanted to do it in one mission, or you could go easier and just do a few missions to build it up

You can see the mission exp payouts here- Exceptional completion gives double the base payouts on things, including exp, Satisfactory is the base payout, Poor is half the payout)


3. Their Proficiency Point assignments.
LD currently has 1HP/1Dex
Midnight has 2HP
Neither of these are optimal for an air attacker or a light healer- HP as a proficiency is almost always reserved entirely for Tanks.

LD as an air attacker would do more damage if she had either 2 strength proficiency (pure offense increase), or 1 strength and 1 dex (somewhat offense increase, but keeping the already present accuracy and critical buffs since she currently does have 1dex)

2Int on Midnight would mean that his damage output was increased, and thus he'd be doing more healing of the party as a whole. A Light healer with no Int profs is a bit toothless in the whole. Healing the party department, because their party healing abilities depends on their move output.
With no changes to him other than swapping his two profs from HP to Int, he'd go from his current 198 Magical Damage up to 306. (which would mean each hit of fairy light would be healing the party roughly 230 HP rather than the current 149 HP, and beam would be healing 153 rather than 99)