This is oozing old Flash game aura, and that's a good thing!
Very cute and very fun game! When I saw the shop allowing you to increase your health, I took it upon myself to progress as far as I could without buying a single balloon. Surprisingly it is entirely doable, and I actually beat Stage 5 with only 3HP. A 1-hit challenge where the enemy has a faster rate of fire and the most health in the game was quite fun. It is a blessing that the player's own fire rate is uncapped, as that allowed me to chuck as many knives as I could where I knew the opponent was going to be.
Great game, I had a lot of fun!