Try out:
Cleric based on Offense, with Fervor and Litany of Pain
Cleric based on Healing, with Heal All and Purify
Wizard based on Offense, with Electric Storm
Ranger based on Offense, with a poisonous bow and Hail Of Arrows
Zombies are OP. My suggestions:
Make them unable to use heavy weapons like railgun and rocket
Mke them run and fire like mad in one direction instead of flipping around and flanking
If I make a sequel, ranged weapons, and different classes are two of the essential additions.