"You've got the look!"; "Let me take a look at you."; "Eye for eye!"; "I've got to look!"; "Here's looking at you!".
The bonelord, a.k.a beholder in earlier versions of the game, will back away from its opponent, trying to keep a distance from the target while using a variety of spells, and usually summonsskeletons which will attempt to surround the attacker. After Update 8.0, it does less damage per turn.
Knights: Chase the bonelord down and attack it. It has virtually no melee skills, so using a shield is obsolete. If you get surrounded by skeletons, a direct path will be difficult, so quickly kill the skeletons and break through rather than trying to navigate around them. Another way is to wait for it to approach behind the corner and then attack it. Paladins: At level 14 or higher, you can just shoot it from a distance with distance fighting of 50+.
Sorcerers: You can kill these at level 13 using the Wand of Dragonbreath and a few mana potions, although this is not recommended. At level 22, hunting Bonelords with the Wand of Draconia should be easy. Just remember to keep using the Light Healing spell as needed and too corner them. This will help to increase the chance of them using their weak melee attack, rather then a magic based attack. Try to avoid being trapped by their summons.
Druids: You will spend more mana at Level 13, because they are strong against ice damage. At any higher level, use the same strategy as the sorcerer.