I think this is a great start towards making your game! The game overall felt good to play. I didn't have any performance problems or notice any major bugs. The only bug I really found was that enemies can collide with the map boundary, so if they spawned outside of the boundary then they were not able to reach the player.
Some suggestions:
1. Since the idea is that you are a weapon waiting for the wielder to show up, maybe have each level have a timer that increases as you level up. The timer would simulate the wielder finally getting to you.
2. Have the weapon not kill all enemies globally, and remove the healing.
3. If you survive long enough for the wielder to show up (timer reaches 0) offer an upgrade to the player. These upgrades could be like: attack damage, attack range, move speed, healing effect, etc. This is what will really shape your game.
Great work on this game so far! Keep learning and making it better!