1st: standard neutral playthrough - found undyne really tough because she kept doing the spears and stuff...I wasn't having a good time. Muffet, same there. Asgore, him too. I mean, I got stuck there because I was like "okay he broke the mercy button" and I didn't keep the pie because I didn't think to keep it. I eventually beat him!
undyne - regarding her, i was determined to not destroy her. i understood why she did what she had to do even if it involved seeing the game over screen.
the neutral route final boss is giygas on heroin. yeah.
2nd: true pacifist - also wanted to save a certain character (IF YOU FINISHED THIS ROUTE YOU SHOULD KNOW WHO I'M TALKING ABOUT OK) and was sad that I couldn't do it. I understood why though!
3rd: no mercy - don't do this run. i couldn't. i got to the last freakin boss and quit for obvious reasons but...still....the entire experience was really hollow and boring.
undyne - i hated having to kill her also. i admire her passion and fighting spirit in pretty much everything.