Board Thread:General Discussion/@comment-26347196-20160410032827/@comment-28615841-20160603004331

I feel that while both bosses are challenging, Sans is VASTLY more difficult. With Undyne, you already know the basis of her fighting. And this is assuming that you have played a neutral/pacifist/true pacifist run before your Genocide. This gives you the upper hand, as you have already experienced her Green Attakcs, and know about her moves, including the yellow spears. She also has invincibility frames, and many options for defense and buffs.

Sans. Jesus. Beating him my first time was hell. After 3 days of flinging myself at him, I was up 45 deaths, and hearing Gaster Blasters in my sleep. I think Sans is more difficult, due to the sheer amount of rules he bends when fighting you. His liberal use of the Blue Attacks and changing gravity make his fight a platformer at times, requiring skill and accuracy to make it through. You are also punished much more for mistakes, as his Karmic Retribution makes sure that you loose a good bit of HP from so much as a nick on a bone. Because of his lack of invincibility frames, your armour does not affect anything. Your items that increase invuln time are also useless. The weapon you have is useles, as he doges it no matter what you do. He also throws most people off gaurd with him having the first move, as well as attacking you in the menue screen.

Thats just what I think