Cool stuff!
I really like how it fits the theme - every wave is the same so you can learn where to shot first arrow or where is the enemy that should be killed first. 
Also main menu with Minifantasy Overworld is outstanding. 

There are some minor things to improve:
1. The walking animation not starting right away (hero slides for a short time and then takes the first step) - I think starting Walk animation not from 1st but from 2nd sprite could improve it
2. (Maybe it is just me) Damage from enemies seems to be hard to avoid when in melee

Skellyboy and Soulgirl! Cool idea for coop! I made it to the end and I am very curious what is on the other side of shiny gate!

I came across a bug where my soul was locked in the wall (3rd level as far as I remember) - I had to restart game and it was good next time. Not repetitive but maybe it was because I moved as a soul from level 2 to 3? Or there is no "absolute" spawn point in every level... 

Zombie driver meets survivivors type of the game? I'm in!

I love the blood splat remaining where a zombie died! 

Things to add that would be a big win:

• Sound control and some variation in shooting sound - currently it is tiring for longer sessions
• Description for upgrades/squads could be beneficial
• Balancing - small zombies are so tough...
• Some environmental elements so we can easily tell which direction and what speed are we driving
  

I like the idea of mixing Jump King style platforms and carrying things 1 by 1 to the predefined point (flashbacks from refueling car in Left4Dead 2) 

Two things that I would be a big win to me: 
1. Music: First, it gives the vibe of "I'm the speed," but in the long term, it becomes annoying—a mute button would be cool!
2. Camera: Smoothing or dead zones would make it more appealing for longer sessions.

The idea and game loop is quite... addictive. Especially when I see known highscore >2k.

It has great potential - from making duels between players to introducing roguelite/deckbuilding mechanics like starting each run with player's selection of towers and upgrades.

Very fun! Enjoyed the twist on dying.