This is very cool! It's a bit rough around the edges and you need some time to understand what is going on and what is your objective.
First quality of life change I would do is to make the attack automatic if I hold the mouse button. As the attack cool down is quite high, it has bad feeling when you keep clicking but only like two attacks, from the 7 clicks actually attack. Or maybe make it attack automatically even if I don't click only requiring the mouse position for the attack direction. Or add more feedback making easier to understand the cool down, like an attack animation, a tiny bar or something like this. Felt a bit unresponsive.
Pause when I'm choosing the upgrade. As the game keep running, you have to rush the decision, which kinda ruins that window of relief while you make a mid-term choice. But maybe, more importantly, this feels like a shoehorned feature. Maybe with time you could find the direction the game is trying to go. This feature is usually more suited to games with 'a build focus'. Like: "oh, maybe this time I will be an ice guy", "now I'll be the poison guy". But I honestly think you should focus on the mask and resource management and that feeling of being the master of puppets for beating a single hard boss.
I died like 3 times to the boss's explosion. And that in itself is fine. The problem is that, when you die, the game ends immediately so it is hard to understand what killed you. I only understood that when I stayed away and saw that ring and then the explosion. On the first 3 times I had no clue what killed me.
In the end, I won with a really cheesy strategy, summoning melee guys right on the face of the boss, and hitting along then, running away when it wanted to explode, then repeat. The fact that you can corner the boss is also a problem, I think, because then melee ends up being very strong. If the boss had a teleport move I think it would promote more variety from the player.
The enemy and mask variety is really cool, but maybe having less units with more variety of effects would make for a more fun and strategic experience. Like a unit that cures the boss, so when it spawns, it have a high priority for the player, because he doesn't want to lose his progress.
Anyways, lots of feedback haha. I see potential, but maybe there is an 'direction' issue here too. What is the fantasy you are trying to create? Hope all these comments help! I really liked the project, and I feel like many players will struggle to understand and quit because of that, which is a shame.
Congratulations! o/