i killed toriel, but i felt bad, so i went back and saved her. a couple seconds later after saving her, i come across this flower and he

knows that i killed her and went back

Undyne is the best boss fight I've had in forever, I'm stuck on Metatton EX and I hate him.

If you're having trouble with Mettaton EX, you might want to load your inventory full of Mettaton-themed items from the Burger Emporium. If you eat those, it raises your rating and makes the battle end earlier.

Another interesting tidbit: If you hold down x then it slows down the movement of your soul. The game never tells you this but it's a good way to do some pixel-perfect dodging if needed.

If you are trying to spare him, you need 10k ratings, but it will only apply after a certain turn in his battle (he'll have suffered damage and yell at you)

Some items that quickly raise ratings by a few hundred or even a thousand are the Stick, Cloudy Glasses from Waterfall, or the branded food from the hotel that was already mentioned.

Also, use your Act menu. Boasting is my favorite, but you have to not get hit during the round to see the most benefits. Posing nets you more and more as you go, especially if you get damaged, and the last one that I forget what it's called raises ratings more for you getting damaged. Memorize when attacks you know are coming and choose the most appropriate action, such as Boasting before the Essay or Break time.