So, firstly, you can just hit the boss, use your infinite jump to go where he can't hit you, and when his invincibility wears off, rinse and repeat. Maybe consider giving him a ranged attack?
Second, the game is very easy. Too easy, in fact. I didn't buy a single thing with the money, granted it only died twice, once with 20 gold, and the second with 30 (I didn't find it worth my time to buy the 30 gold upgrades), so even without any upgrades, the game was still a walk in the park.
Maybe give more of an incentive to use the gold, or make the game harder? Most enemies if I didn't want to fight, I could just jump over and get the powerup, and leave (I did go back for them because I'm a completionist).