That's because it IS impossible and you DO need to cheat. You deal barely any damage to her when you get the opportunity to attack, and even then you barely ever do since you constantly need to be healing and cleansing status effects.i can take any advice to beat her even if it's cheating cause the fight looks really impossible
So, my advice is just that: Cheat.
Alternatively, if you want a version of the game where the genocide fight is ACTUALLY possible to beat, I can give you a link to v7.02, the last version I have saved where Genocide was able to be cleared. However, the genocide route is still MTL at that point, so be aware of that.