This seems fun! I can imagine trying to kill zombies faster so the boss shows up, you have more aggro than the opponent, the boss comes towards you... I wonder how easily can you break this, if you might easily end the game with every miniature with about 100 aggro.... If you can build an engine of 3+ rifles you can really create a meat grinder.
I feel you took more or less my path of design, instead of lowering the amount of elements, you increased the amount of stuff that happens for each element.