Arcane horrors are easily killed, but have very high damage output. When they are immediately attacked, they are easily dispatched, but left alone they will devastate a party. It is a good idea to ignore other enemies and attack the arcane horror first.In a party with ranged attacks, it's possible to kill the demon without it even damaging the party. In a party with a mage, using spells that drain mana like Mana Cleanse, Mana Clash, or Mana Drain can severely inhibit the arcane horror, especially when used in the improved drain Spell Combo. When fighting the arcane horror in the Werewolf Lair, it is easy to kill it swiftly if you can get all party members a ranged weapon (either a bow or a staff), and keep them on the stairs above the arcane horror. Make sure to have hold position on however. All four party members must stay near the door.Send one of them, an animated dead, or a summoned wolf near arcane horror and immediately run with it to the party. The arcane horror will attack that party member with spells like paralyze, weakness, or etc., and a horde of undead will follow.After killing the undead, the arcane horror changes his position and you can target him from the stairs.Alternate strategy: 1. Kill off the minions around the horror. 2. Select whole party and then move em way back up on the stairs or so, this sometimes drags the Arcane Horror to follow you there. 3. Now away from the platform-like thingies, the Arcane Horror is unable to teleport, making him an easy target. See: Dragon Age: Origins