The current problem is Thief tree, 5* Warriors(currently 4* warriors are stronger than them), and Guardian tree (light exception of Blood Knight).
For PvE, there's no point on using Melee classes on other maps than Gold Event.
For Legendary quests, even 1* monsters to "meat shield" mages, priest and archers.
Currently due to MDEF (magic def) being included within the MAG stat there is no loss/downside to priest and mages.
Archers has range so they are able to just build ATK/CRIT and rain pain to enemies.
Warriors have Horrible DEF and MDEF so they just die instantly to anything an arrow/skill from Archers, Spit of bugs or Mage spells.
For most Melees they have it the worst, since it is like an automatic "Kiting" position for them by ranged enemies (PvP/PvE) and their horrid MAG Stat they get one-hit KO-ed by any skills of Mages/Archers/Monsters. They would not even last to use any of their skills as a tanker, stealth or warrior.
My suggestions:
1. Is to give melee classes like thief and warrior a passive that parries spell 1 time per turn. (Abusable)
2. Is to give melee classes an Extra Move (Just Move) so in a turn they can either Move 2x or Move 1x + Attack 1x not Attack 2x. (May be improved)
3. Separate MDEF from MAG as an independent Stat itself so to not make Mages and Priest abnormally hard to kill (they're supposed to be squishies)
4. Lessen Skill Cooldown of Melee classes skills or add an additional conditional to counteract Low Skill cooldown for example: using HP as an extra cost, getting poisoned from using a skill, etc... (May lower skill damage in-case of spam)
5. Increase Stats of Melees specially HP, DEF and MAG.
6. Nerfing Mages will just make them unusable and will have the same problem like melee classes in the future. But giving Mages Higher skill cooldown specially lower rarities will make the battle more of an even playing field.
7. Increasing DEF usefulness, currently DEF is the 2nd weakest Stat second to HP. Since DEF only reduces physical attacks and not CRITS or Magic Skills there are very little instances that you would need to increase it, for example a Kratos Victoria or Darrion since they would instantaneously die from a Skill.
All of these are just suggestions maybe you guys are intending to make Mages and Priests the Main class at arena we wouldn't know.
That's all! Thanks for reading! I will add more suggestions if it is useful.