Some neat ideas and the music is catchy. :)
It's not immediately obvious that the health bars are also the indicator that the player has locked onto a target, and the very short range in which locking on is possible makes it more difficult to use. On the smaller enemies there's no motivation to use the gun when a few steps closer lets you hit them with the stronger mana blade. This is fine of course, if there are many enemies and some are better targets for a certain attack strategy. But since there are only these minor gun enemies before final boss it gives the player the impression that the gun has little to no worth and they're less likely to try it on the boss where it's necessary.
Speaking of the boss and talking more about that gun, it would be nice to keep the lock-on in place even when out of range, with some sort of indicator showing whether you're currently able to hit it or not. It can be as simple as changing the color of the health bar, or a target icon which is crossed out when you aren't in range.