Aran does not have a controllable agro list, he rarely melees and his spells look to random target players.
Aran’s health must stay below his mana, if he runs out of mana before killed he will polymorph the entire raid and kill everyone, fortunately being a caster physical damage is incredibly effective against him.

Encounter

Aran is an Arch mage and casts spells from all school, his basic Fireball, Frost Bolt, and Arcane Missles can be interrupted or counterspelled. This will help in reducing his damage output as well as stopping him from using mana.

Aran has 5 special abilities that he can cast at anytime, they can not be stopped.

Flame Wreath – when he casts this spells a targeted player will be surrounded in flames, no one can move in or out, if he casts Flame Wreath nobody moves until it is gone.
Blizzard – a large AoE Blizzard that rotates clock-wise around the outside of his room, the center of the room where melee is will be unaffected, ranged and casters can run around clockwise to avoid it or if all else fails run to the center where it will not affect.
Arcane Explosion – Aran will summon the entire raid to him and slow everyone, this slow debuff is removable. Everyone must run to the wall to get out of the way of his explosion, it is a 10 second wind up; even slowed players if they move immediately will get out of its range in time.
Counterspell Aura – Aran spams an AoE counterspell that affects everyone in melee range, it may be an Aura.
Chains of Ice – 10 second root, can be dispelled.

At 40% health Aran will summon 4 Elite Water Elementals, they can be feared and banished, after 90 seconds they will despawn, during this time all DPS remains on Aran as he will remain unchanged.

Party Roles

Main Tank / Off Tank
DPS Aran

Melee DPS
DPS Aran

Ranged DPS
DPS Aran, avoid Blizzards

Healers
Heals on entire raid, avoid Blizzards