I like the idea of incorporating exhaustion. How about this as a general idea. A con save with a DC that increases every time the target is shot with a (or perhaps the same) ZAT in a minute. And having the degree of failure indicate the level of effect? That would allow an over time debilitating to hardier beings like the Wrath or times we've seen some lofe form being slowed down then KO'd or killed. "Mooks" with their low saves would be affected more and players would then still need to be aware of ZATs on the field but not be afraid of instant death outside of multiple hits or unfortunate rools.