Amazing update but I do have some feedback with regards to the no-damage-taken scene. The flow of combat, just like in actual Undertale, is generally a very satisfying flow where if you take damage then it is generally your fault. But there are some cases where if you get unlucky with attack placement, it can be literally impossible to not take damage in a given scenario which sucks because of the added reward for no-hitting the boss. In the previous version, while a bit annoying, taking unavoidable damage wasn't nearly so bad as it is here because there's an actual reward on the line that RNG fucked you out of, and not in the sexy way.
The main offenders are the attack with the rows of knives moving against each other, and the various attacks that swap to a black background and pull your cursor towards Chara.
With the rows of knives, normally they're an absolute cakewalk, but if Chara adds in one of the big slashing attacks and you're on the opposite knife track, you literally cannot avoid taking damage. Either you swap to the other lane and take damage before you can come back up on the other side of a knife, or you stay on the track and get dragged to the edge before taking damage from either a knife or the hitbox of the slash attack that lingers just a few milliseconds extra to feel like that shouldn't have hit you.
With the black background attacks, they're normally fine, but sometimes with mouse controls you'll let go of the button to quickly reposition your mouse because you've hit the edge of the screen, but when you click to start dragging the soul around again it occasionally just won't register that click, which leaves you lingering long enough that by the time you click again and regain control, you've already taken a hit.
One more that is far less egregious, but annoying in just how swingy the attack can be in terms of difficulty, is the attack with the lasers you block with the knives. About half the time, most-to-all of the lasers will be more or less perpendicular to the knife wall, meaning you can more easily move to block them as knives disappear. But the other half of the time, some of the lasers are nearly parallel to the knife wall, which can put you into what feel a lot like lose-lose scenarios, even if they are technically possible to avoid. Because you need to change which side of the knife wall you're on to avoid that specific laser, which can be very frustrating when your movement speed stops you from getting back to the other side before the lasers perpendicular to the wall start firing again.
Overall, 9.5/10 update, but the RNG fucking you out of a no-damage fight because of something you literally couldn't avoid is incredibly frustrating when you're grinding for that scene.