when a monster's hit is calculated, it also calculates a random number for how many times it Combos in that hit, between 0 and the "max combo" stat for the monster. Monsters also have levels, and, some monsters have their "max combo" stat increase according to its level. What that means is that there is a lot of randomness in how much damage each hit by a monster will be. It could be 1 damage or 20 damage.
Anyway I only wanted to explain that because, the note monster is actually a lower-level monster than the piano monster typically, although they both appear in some zones. In later zones you only get piano monsters. You have a defense stat, but it doesn't do much against combo hits because it just subtracts from the damage you take. So if you have 7 defense, and the monster has 6 attack and does a 0 combo hit, it will do 1 damage (any hit always does at least 1 damage). But if it does a 3 combo attack, it'll hit for 18, minus 7, will be 11 damage.
And as for healing, you do passively heal while walking around.
Thanks for trying it out!