Large skeletal humanoid
The Bone summoner has one attack and one ability. To attack it launches a fast moving skull projectile which has strong tracking. Its ability allows it to summon wave's of Bone Rangers and Bone Sentry's which will not drop experience gems but can provide mana and health drops.
If the Bone Summoner is isolated it is best to rush the target and use your more most powerful weapon to rush them down. If this is not possible keep your distance and take advantage of freeze or poison effects to whittle down the health of the target while keeping out of danger. Should be made a priority target in rooms with no other spawning minions (Excluding Obelisk rooms)