You stun them with the cool time powers of your awesome new hammer, and then dispatch them swiftly.
Author commentary: John just got this killer hammer upgrade, so of course we should get to see him take it for a spin. We've never even heard of a lich before, but it stands to reason they are a lot deadlier than mere imps. But John makes light work of them anyway with his new hammer. So, yep, it's a strong hammer all right. Good to know.