Instructions
Set your pet dragon's element to Ice; don't confuse this element with its Water element. Remove all the members from your party (the dragon doesn't count as one of them); extra party members make Akriloth stronger.
Wield the Frozen Claymore weapon, which you should get in Battleon at the start of the "Akriloth" quest. This weapon will deal more than 200 damage points to Akriloth with each melee attack.
Use the Seed spell to charge your attacks, and use Summon to bring extra minions to fight with you if you are a Necromancer.
Devote a turn to healing yourself if your hit points get depleted; let your dragon and minions attack by themselves during this turn. You'll especially need to do this if Akriloth assaults you with a fire breath attack
Re-summon your dragon if Akriloth destroys it.
Continue battling Akriloth with the Frozen Claymore and your summoned monsters until the dragon goes down.