There's a number of good options to improve the bats, I think. Atm they hover at whatever position they approached from, which means they're never at the same angle twice. You could have them prefer remaining at one of the 8-way directions to the player perhaps, or exchange their projectile for a wide horizontal swoop attack so players have a chance to hit them as they approach. Future potion upgrades could also explode in midair to flak-cannon more agile flying enemies, perhaps.
As for the aiming/binds - Treating it like a twin-stick shooter (Binding of isaac and metal slug also come to mind) is probably best.
- Movement stays on WASD with other utility keys being proximal to the left hand (Q, E, Tab, R, F.) I'd personally put heal on Q, but that's subjective.
- The player aims toward the cursor's global position, allowing players to move and aim independently
- throwing a potion is on Mouse 1, and you can maybe have mouse2 for potion swapping?