This is definitely a very neat and stylishly animated game, I love the concept and the art style. However, I don't know what current plans are like, but I'd warn against making too many bosses that are just big unflinching damage-sponges.
These sorts of bosses work for Smash because they provide a style of gameplay that every character can approach in the same way with less risk of bad matchups, where everyone's moves have the same effect (just do damage until you win) and everyone can avoid attacks with the generic dodge/shield/jump moves. But... because of this, they're too far separated from the standard gameplay, and somewhat homogenized. More of a fun sideshow than a culmination of the mechanics.
Since you don't have to work with this constraint, you should be able to make bosses that follow the mechanics of the rest of the game, and provide more satisfying "make use of what you've learned" challenges rather than basically making the player adapt to a whole separate playstyle for boss fights