Was fun, though very repetitive.
Would like to see more enemy variation/attacks, as well as arena layouts.
You do get overpowered quite quickly and some upgrades like mana and max hp didn't make much sense to upgrade after the first little bit, as you'll have more than you'll ever need of the latter, and the overcharging of your HP via magic makes upgrading it pointless.
For balance, it might be a good idea to scale the HP gain from magic to cap at your max hp value (or some% amount over) and to start scaling the mana cost of spells higher the more points into hp regain and magic damage you have.
One problem I did have was the sheer amount of mashing to move around. Might just be getting old but it was pretty uncomfortable.
Perhaps consider keeping the character moving until they stop holding down the movement keys, at least as a toggleable option, maybe for the weapon swing as well.
Shop music gave a nice atmosphere to the area, though the main music would have benefited a lot from a longer track, and possibly a new or remixed version of the song once you reached different slime colors, to aid in that feeling of making progress.
Hitting the enemy felt very inconsistent, not sure if it was a misleading hitbox issue or hidden accuracy calculation?
Felt like something that'd happen on the console, but didn't feel good to play with. Once you outscale them with your healing it's not a worry, but still does make it feel clunkier than it should.