The tutorial is hard to read on the game's title screen. Maybe give it a solid color background, or a window.
Character design is great.
The gameplay itself is impressive, and actually I played a game kind of like this last night called Idle Champions of the Forgotten Realms. For me, two things stand out to fix. One is that the game, at least initially, is so easy for the fighters that your healing isn't really necessary, so some game balance would be good. I see that balancing is an issue, and part of that might be that you spawn with all your allies. The game I played last night made unlock your allies as more enemies attacked. Just a focus on pacing in general (maybe start with one ally and one enemy type, and ramp it up) could help.
Second, I think the visual space could be somewhat better designed. The background could be upgraded (not really sure what we're standing on), and the camera could be moved back a bit so we can see more what's going on. I don't really know where we are or why these people are attacking, so besides the phenomenal jokiness of the character design, I'm just never really sure what's going on.
But I do think as sort of a prototype, this is pretty cool. Maybe focus a bit on what you want the experience to be, what you want the player to get out of it, and I think you'll have a pretty successful thing. You've already got the basics down.