Really happy to see a complete battle system created from scratch for this entry! Still plenty of work that can be done to expand the battle gameplay of course, but a quick and easy tweak to improve the gameplay flow would be to move the HP counters and targeting buttons down closer to the enemy/player sprites. Ideally the sprites would be placed over the targeting buttons so that you click directly on them to select the target of attacks and healing abilities.
Also I'm not quite sure how to describe it, but I really liked the wave and water shader used during battles and how the waves were animating so quickly, like it felt like some kind of manic Spongebob diorama made with paper cutouts. If you added just a tiny bit of animation (even just to make them wobble or squirm around slightly) to the enemy/player sprites it would have a really neat cartoony visual effect that would immediately grab people's attention.