Board Thread:General Discussion/@comment-44447064-20200213214202/@comment-39385741-20200214142252

I personally think Undyne the Undying is harder than Sans. How I defeat her:

When you're in Snowdin, get as many Cinnamon Bunnies as you can. (They're free in the Genocide route)

In Waterfall, buy the Cloudy Glasses and Torn Notebook and equip them. Then buy a single Sea Tea. (This will be useful for when Undyne puts you in red mode)

Go to the box near the shop, and make it so that your entire inventory is filled with Cinnamon Bunnies except for one Sea Tea.

When fighting Undyne the Undying, make sure to only use the Cinnamon Bunnies when you have low HP. This fill make your Cinnamon Bunny reserve last longer. Use the Sea Tea when Undyne puts you in red mode.

Finally, keep trying. For a really long time. Eventually you'll kill her.

Or, you could just not do genocide in the first place. That's still a viable option.