Somehow i managed to defeat the zombie horde in test area, level 20. You would be amazed at how much damage someone with only a pistol and 120 barrels can do (my ammo for the uzi, assault rifle and grenades was all spent).
hint the devils can also set zombies on fire. this helps if there's a large horde around them. u can also set barrels around them and they will kill themselves if they try to attack you because if you dodge out of the way they blow up the barrels
As far as i know, you could make a better mmo by making many more different quests like collecting or delivery. if the bulk of the game focuses on the story missions it starts to lack any replay value. Make way more quests and more variety with more characters than just Bruno Paine and everybody in story missions
its smart to first send 3-5 mice and then send some Hoods for backup. after that you can get some more powerful units as the first attack buys you some time.