He's usable in every stage where Balrog is usable with very few exceptions, then you have: most enemies that he outranges, stuff like sunfish, all LD enemies, most of those deadly bosses... he may need a push in terms of levels to get tankier, but he's one of my most used units, as well as one of the strongest.
There's such a thing as APKS. These can serve as different versions of the game, but there are no modifiers for the original version of the Battle Cats. In fact, the game detects when exterior programs act upon it.
being quite a good unit genraly (in my op), i think being bad against zombies would make him not too op. (i think having slow movement might help block zombies from your base, while other units make them burrow, not being to bad against them)
there is already the othe legend rare if u r looking for a good tank for zombies