So, I may be crazy but...

I found Undyne (both her pacifist AND genocide fight) to be... not that bad. Maybe it's just because green SOUL mode is easy for me, but I really didn't struggle THAT much.

So, I had undertale on my PC, but at one point, it needed some repairs. So I downloaded a mobile port temporarily. I beat her on my phone after 3 hours or less. On a phone.

I think only the red attacks that are kind of troublesome. Eventually, though. I was able to outlast her.

I used a special strategy, by the way. For "Phase" 1, which is for me until the red SOUL spear circle attack, I used the Old Tutu for more defense since I got used to the attacks, so if I mess up once, I'll take less damage.

For phase 2 (at the yellow arrows part), I used the Cloudy Glasses (Or the Torn Notebook I don't remember) for it, because they were slightly harder and I didn't want to get hit repeatedly.