Dude, i did it in phone and it took me like 5 trues what 😐 (i'm not stating you have skill issue, i'm just stating that the battle was easier on phone or i had a lot of luck while dodging, not even the best items or the best armor)
To be honest, when I played genocide I actually personally thought that Zenith Martlet was easier than the fight against Ceroba on the genocide route. I had significantly less trouble beating Martlet.
Actually, in the undyne fight when green soul, you don't actually move, you just block her attacks, moving the spear to block them, but your soul doesn't move.
My first time fighting Sans, it took me around 50ish attempts. On my second playthrough, it was only 3. From then on, I could first try him. Today, after a few years of not playing, I decided to finish up an old genocide run. I beat Sans on my third try lol