Bone Horrors have a large amount of health and can deal high retaliation damage as well. Though they lack any form of special abilities, their normal attack has some armor penetration and can still hit very hard. They are very slow though.
Pincer Grip: Deal high damage to one target, reduces target's action clock.
Any attacks against it will hurt your characters so it is best to use a large single attack on it rather than a series of medium ones. Though it hits hard, it is also very slow making any slow spells reducing its damage over the battle immensely. A Cleric's Holy Light is perfect for this and will keep him safe from retaliation since Holy Light does not deal damage. Due to this, it is best to kill the Bone Horror last. If this is not possible, Inquisitor's Fervor, Marksman's Eagle Eye'ed attacks, Rogue's Hide in conjunction with attack, Sorceror's Incinerate with the Empowered stance, and anything with a Cleric's Anoint are some good methods of dealing a large amount of damage with one attack.