The art and music were really nice! Aside from that the game is really simple, and as others have already mentioned, it's impossible to lose.
Some things that could've made it more challenging/ strategic would be to give the boss different abilities to use, and options. Not just who to attack.
Perhaps also if part of the game was to direct your minions in the dungeon before the boss-fight to wear out the hero's party before they reach you. The better you place them, the more worn out your party becomes.
I believe these could add more depth to the game.