.

use sea tea
use notebook and glasses
memorize and learn her attacks and patterns
learn how to dodge and react quickly
should be easy after that

I know how to memorize attacks I'm pretty sure I no-hitted Queen before, but Undyne's attacks are in a new plain of speed.

pretty much, you pretty much have to be really fast and good at dodging
that's why I recommend sea tea, sure it only heals 10 hp but it gives you a huge speed boost
and the notebook and glasses
for me the damage doesn't change much for the notebook
but for the glasses, you have to make sure you actually don't get hit as often, even if you do get hit, you'll be invincible longer to get out quickly