This boss often causes death for unprepared players, even high levels, so if you are planning to hunt in Vampire Crypts, always be all ready in case this boss appears. Arachir often hits with paralyze + drunkeness combo, so you should always keep a Dwarven Ring ready in case you need to escape or you try to outrun it.
The best way to kill it is a simple formation of 2 Druids healing the blocker and shooters killing fast, with Fire Waves, Fireball runes and Divine Missiles.
Mages: A possible strategy for mages would be to lure it to the rope hole while using Mana shield. Then use either Hell's Core (Sorcerers) or Eternal Winter (Druids) to kill this vampire lord and it's summoned Liches.
Higher levels can successfully kill this boss alone with help of field runes like Energy Bombs or Energy Walls, to keep summones Liches or eventual Vampires from attacking you.