Board Thread:General Discussion/@comment-39394247-20190508143132

Undyne the Undying appears to be weakened by her determination, so that she has less HP than normal Undyne (1095 vs 1500), and if hit by LV 10 Frisk, always takes at least 29 damage.

Apparently, when you hit the FIGHT button, you either MISS or do "600" or more damage. There is also a hidden x21 multiplier for the damage values as well, as evidenced by the fact that if the damage is higher than 666, it is always a multiple of 21 (1071, 1491, 1512, 1638, etc.) 