Bug(?): Melody Miestro goes into negative Strength
You need to prioritize landing as many hits as you can in the first few turns, then finishing it off with some unblockable irreducible damage. Take as many spins, fire strikes, Adrenaline, and rollerblades relics as you can for the first part, look for Planned Moves, Asteroids, Mirrors, dancing shoes relics, and (in a pinch) corset relics. Failing that look for big attack cards, Moonwalks, and Muscle Shirt relics.
Moreover, even if you build correctly you can be easily shut down by opponent playing Butter Magic or Mirror. Don't worry, it's a normal part of playing this hero.
And no, strength going negative is an intended behaviour, just like in Slay the Spire.