The game looks quite promising. Good work!
Gameplay wise I would agree that difficulty isn't quite fair. My nerf proposal would be to allow player to rearrange their zombie formation, so you can get your gunners out of harms way easily without trying to reposition your meatball to make them spawn in a middle while aggroing ten packs around you in a process. This can probably be done by adding some fixed formations. If taken a step further and allowed to move units individually the player can also move out their armored knights in the direction they are needed the most. It's a more engaging gameplay but probably will make a game too easy with the current design.
P.S. It greatly bothers me that the walking animation of MC continues when he runs into a wall.