The easiest way to beat the final boss is by using the fact that he can only attack you when hes next to you to you advantage. Simply be above or below him and right before he is next to you move slightly closer and start attacking. I didnt get hit once doing this
Best path for me was simply all 4 of the final upgrades, it will put you at 1 below max of all stats except weapon, which is 2 below. I was able to get the Hard badge this way