Beat the game with just removing and upgrading the starting dice (I added a boost partway through and removed a lower level starting boost but that turned out to be a mistake). One attack, one defense, the starting heal die, and two boosts. Every turn you play the attack and defense dice and the highest boost. When you have enough defense you get both boosts to three and then play them on the heal die to heal back any health you lost. Worked fine, though I was losing health rather than gaining it on the boss and the last two fights before it, just not enough to matter.
What's the weird black shadow after you beat the boss? It just appeared with hollow wind music and then the end screen played...