Really fun, I love the idea of having to directly trade in health in order to do damage to enemies, as well as the "using pebbles as currency" mechanic.
Though I have found some problems with the game. Mainly, it's a bit tough to actually hit the boulders when they roll around and it feels like a lot of my hits aren't being registered properly, it feels like most of the time I end up accidentally shooting right above them, not triggering a hit, and I feel like that might be because of the odd perspective. I've also found it quite annoying how the pebbles will sometimes bounce off the edge of the island making you permanently use them, but other times they'll just bounce back; I'd suggest not letting them bounce off the island as it made me hesitant to hit enemies at times as I didn't wanna lose my pebbles.
It might also be fun to change the camera perspective when the shop keeper comes in, to focus more on them, but that's just a cool tiny detail you could add in.