Great use of a single bullet! Feels great to brutally smash the poor treasure guardian.
For the gameplay I would recommend making it easier to direct the bullet. I found it a little frustrating to control it. Maybe letting the central point of the aiming be wherever the player clicks the mouse would make it more satisfying. Also, for some reason the game got really laggy for me starting on level 3. Could't get past those spikes because of this.
Overall great job! :D