This game is really well made and polished, and I like the concept, but I found it really hard, until I discovered that just mashing click makes it trivial. I went to level 10 with my health always at max, while I struggled to get to level 4 without spamming. I think the main problem is that there are a lot of enemies, and it does not matter if you align perfectly your shots to kill two enemies at once, because 5 enemies have spawned in the meantime and you could just have tossed it and maybe killed one at random, plus gotten a dash afterwards and got to the heart that would have depleted otherwise.
But don't get me wrong, it's a great game, it's just frustrating that the optimal strategy is the easiest one to pull off.