Hmm. I see your point and i know about it. It would be really hard, but it's possible. I'm just not sure how to fix that issue. Maybe every round there should be ranged creeps, dunno. Do you have any suggestions? I really need suggestions about gameplay. Perhaps low hp should also cause something like "Fatigue" just like low Stamina causes, which reduces your movespeed, or, being hit may cause you to slow, i'll think about it.
Fatigue is good idea, Also you can give Slow ability to few zombies.
Add one more player to map, Add new weapon like mage staff and make zombies a little harder.