Seems like a good start, but there are some issues, especially with control complexity.
1: Why are the cards standing up. They are cards. They should lay on the ground face up. As it is, I spend most of my time looking at the back of the cards which is extremely unuseful. This would also make them easier to click on.
2: Why do my offensive skills work when I don't have a target. This just makes me waste time waiting for a refresh. Either fail to activate without a target, or target the closest enemy.
3: My units keep randomly deselecting when moving, getting me into bad situations. Clicking on the ground should always move, never deselect.
4: Make enemies appear earlier. I don't like stepping into the circle only to find myself in the aggro range of multiple enemies.
5: Make the camera lock to selected units. Also have default zoom just barely large enough to hold one arena.
6: Simplify the controls. Many of them are too fiddly and unintuitive. And there are just too many of them. I should not have to check a controls list to figure out how to do anything.
7: Exp gains are much too slow. I completed the five parts of the first area using basically one card and only got to level 2.5. Wasn't even enough to unlock the second skill. Maybe make them get exp for doing what they are supposed to do. Caster gets exp for casting, support gets exp for healing, tank gets exp for tanking, etc...
8: They are three hero cards. More would be nice for different attack styles and synergies.
9: Something to do with gold once all characters are bought. It's pretty useless atm.