I never had any problem with Bonecrushes, even at low level. Ghail, my cleric, has been able to fight them since level 5. Zidonne, since level 4. Gavaine, who is an archer/ranger and dual weilder, had problems with them until roughly level 7, but after that has never had any problem with them. But you have to be ready and have a strategy, you can't just rush them like everything else. Called shot goes a long way.