Hey I played your game! Here are the answers to your questions:
-There were a few frustrations. One big one I think is, when I would cast a spell, it wouldn't instantly go. I feel like that's the core of the game, and should be really satisfying to do, but clicking and not getting instant feedback is kind of annoying.
-Also, I didn't like that there was a cooldown and also some sort of resources keeping me from doing spells. I think you should pick one or the other to simplify things. It's frustrating to see the icon become available, click, and to be told I can't click. I guess you could keep both, but don't make the icon clickable if I can't cast it.
-One other frustration was the selecting heroes for single spells. I would select one, the hover over another and it would switch. Making this more intuitive would be good (even like scrolling through with a button might work?). There might be an alternative I wasn't using, but using the mouse was a bit frustrating.
-I liked figuring out the different spells and the progression. I think leaning into the healing spells, making them more satisfying, and make upgrades really interesting, will be a good direction to go.
-Maybe make the heroes smarter? Like why do they just stand in the middle of a trap and get hit? Like the second level with the blades, the guys just stands there. Maybe make him move?
-I don't know if it makes sense to have the healer be part of combat. The combat doesn't feel that great anyway, just spamming space bar in the middle of the scrum, so I don't know if that's necessary. It might be better to just focus on the healing. I kind of like the dash, and that it becomes part of the healing as you gain abilities. Giving the player a reason to move around is good, but I'm not sure if the combat is a good reason
-I had no idea what the non healing spells did (dissipation or something?). I guess it had something to do with countering the enemy, but I didn't really understand what was going on with it. I think something like this is interesting, and a way to make the game more fun is to have interesting enemy designs that you need different strategies to defeat. But that probably needs to be telegraphed a lot so it's really clear to the player what they can do. It would be interesting to have different spells provide different buffs to your party, and using them have different effects on types of enemies. Like for an armored enemy, making your team stronger helps. For a magic enemy, you can give them immunity to spells. Something like that. And then depending on the build as you go it determines how you'll deal with next enemy, so giving the player some strategic choices.
-The cool down for some of the spells is too long, I think.
-The one thing I would change is making the casting spells more intuitive and satisfying.
I think you've got an interesting base with a lot of potential! Let me know if you have any questions about the above.