On every character there is a certain area of the box where you will get hit the least.

Dummy: Just kill him or spare him depending on what run you are doing.

Flowey : The very top of the box is best because you will get hit by the bubbles faster.

Fakit: Try to stay at least near the bottom of the box in both attacks.

Gello: The bottom right corner just be sure to dodge the bubbles coming from the upper-left. Stay in the bottom right corner for the second attack also until the bubble radius starts to get wider then when the bubble in the middle spawns go in between the two and move with little movements as to not lose HP.

Potatoid: I found that the bottom right corner or the upper left corner are the best places to be depending on where the fries jump up from.

Nabstablook: Upper right corner will dodge everything enough said.

Mightgasp: Bottom right corner is a very nice place just be sure to dodge accordingly

Toriel: Bottom right corner is ideal for her RNG attack but for her other attack just move around using Up+Left, Left+Down, Down+Right, or Up+Right. She dies automatically in genocide if you remembered to kill Fakit.