Boss design - Poor design, its basically copy of regular leech/sorcerer, who are in turn the worst designed enemy and with their teleports you cant even get proper look on them. So you take the cheapest enemy and make boss from it, not good.
Make him look more glorious, some mouths on his chest, those thingies with skulls on his shoulder, make his staff and body glow more and make flies/gas around his body.
Arena - (people who make games dont play games - anon) You get caster boss who does nothing and you are only supposed to dodge its spell. First problem why there is no cover. Second problem with the arena, why there are unreachable places for players where the boss can hide and why can he leave the arena.
Make the arena bigger, big empty square, surrounded by several buildings you can enter and move to higher ground. Add cover, some pillars and carts.
Abilities - One cloud that does basically nothing, I believe most people dont even know it exists. And one super imba undodgable AoE that simply hits you if you are not lucky enough for him to teleport with his back to you.
If the arena was bigger players could position better, spells could be much stronger because now you have where to hide, so if you get hit its fully your fault. Add cover, thats basic thing against caster boss, be it pillars, overturned carts or different floors in buildings. Give him more abilities, instead of sending small groups of enemies, he could revive some adds he has in the arena, rothelms, stormvermins. When you add cover he could summon pools of something to counter that cover, so you have to react and move to different place or choose between direct hit or DoT from puddle. (All things are “How to make boss 101”)