Sker Ritual

Issac is the main boss on The Ashes of Sker Hotel. You can “kill” him in the normal zombie rounds, but this will not pop the achievement. You need to finish all the story objectives on the map which will eventually lead to a “defeat Issac” objective.

The first priority in this fight is to kill the electric zombies that spawn with him. They do not seem to keep spawning (at least on normal mode). Then, focus fire on him while avoiding his electric attacks. The only other thing to keep track of is to not stay in any room that are being flooded with poisonous purple gas. It will quickly take your health down. Defeating Issac ends the “story” objectives on the map and allows you to extract from the level.