So here's the deal; I just tried out ZM solo on the easy map and went for the mercenary. His combo power is already a bit OP, but it's also self-replicating. This is most notable when you max out the combo power duration upgrade. Once you activate auto-aim, it will work as normal. The thing is, the actual time the auto-aim is in effect lasts longer than the time it takes before you can rack up a combo again. This means that, at the end of the auto-aim combo, your auto-aim can get the five kills necessary for another auto-aim. This is incredibly easy to do, and let me get through all normal days simply by hitting E and kiting (with some liberal use of the right-click ability as well). The only thing that stopped me was the boss, since I couldn't direct the auto aim and wasn't familiar with the attack patterns (as well as just being bad at ZM, most likely, though I did get it down to about 1/4 of its total health).
Obviously, this is not only incredibly OP but it's not nearly as engaging or as fun as a proper ZM match should be. First things first, the cooldown for the combo ability has to at least be as long as the combo ability itself (if not longer than). I'd also highly recommend changing the auto aim to use whatever gun the player has, ammo and all. Whatever guns I'm given are far too effective and have practically no risk; using the player's current weapon would force them to at least be conscious of their weapons and ammo to keep their auto-aim ability effective.