His rez takes 2 tiles to kill post rez or one skill use. Because his ability drops when he dies it makes it nearly impossible for him to fire a second time.
On contrast when Alby or MN revive someone that person is revived with a heal and often with full mana (because if you tiled them to death they probably charged). That is what makes their revives so powerful.
If atomos was revived with 50% mana, kept the ressurection buff upon death, and the buff was undispellable, he would be in the proper place I feel.
The above changes leave him slow, dont make him a huge threat, but if he fires you become unable to kill him for 5 turns no matter what you throw at him. This requires you to play around him and gives him value as a slow hero.
The 50% mana gain on revive adds punishment to killing him while his skill is up allowing him to fire in 6 tiles. His damage isnt and shouldnt be the focus, work on his utility to give value to him as a slow hero.