Take out the blue crystal first. If you don't, it will restore the other crystals and raise the lich as a skeleton. Next, destroy the white crystal. This one is responsible for healing and regenerating. Once that's done, destroy the red crystal.

The lich isn't too difficult if you follow a plan. If you have silence scrolls, use them when she transforms. Her spells are lethal, but if she can't cast, she is absolutely harmless. If that isn't possible, do your best to take her out when she's in her initial form. While she can attack with her fists, she doesn't do that much damage. Beware her charm spells. She can only cast four, but they can cause havoc on your group.

Finally, focus on her head. If you hit it with enough damage, she will go down. Try to do this when she hasn't transformed. This will depend on the strength of your characters. If she does manage to transform, do your best to always target the head. You want her dead as soon as possible or your people will be in a world of hurt.