Type /noshift in the chat window when facing undead opponents. By using the Heal skill, you will be attacking them with it. Note: Do not try this on enemies that are not undead, as it will heal them. Hold [Shift] to heal yourself while in this state.

Type /ns in the chat window to enable reversing heal. With it, you can use the heal skill to kill the undead type enemies. Your in-game mouse pointer will now be a sword when you move it over players. This may make it difficult to move around or run away.