Sans and other weaklings =
10-B,
10-C with weaknesses (Scaling to physical stats of a training dummy)
Ruins monsters, minimally determined frisk, flowey and snowdin monsters =
9-C to at least
9-C,
10-C with weaknesses for monsters (Frisk should be comparable to chara as a child who survived a lengthy fall with nothing to cushion them and seemingly minor injury. Royal guards use street level weapons. Flowey almost kills frisk at the start of the game. Frisk is also a human posessing more physical matter which is what makes humans so canonically stronger than monsters. Hed be weaker than average humans as a child but should still at least be comparable to average monsters)
Papyrus, waterfall, hotlands and core monsters =
9-B, (Hotlands monsters use explosives, papyrus scales to ice wolfs ke. Mettaton destroys a wall, though even ignoring that since it may have been a type of stunt wall pre planned to be destroyed, they did shake the ground a bit beforehand)
9-C when refusing to fight (Jerry can be damaged by frisk while theyre at this level, papyruses stats in geno go down to 3 when sparing frisk though frisk being genocidal may also lower it) down to
10-C against genocidal bloodlust (Muffet drops to -800 late genocide route. And even early genocide bloodlusted attacks do
far more damage than an off guard attacks)
Undyne, asgore, toriel and mtt =
9-B (Toriel was gonna destroy the ruins entrance. Mtt takes yellow bullets from frisk that can split his mini robots. Undyne can break concrete, shake her house, split a table in half as well as a bridge, and was somewhat damaged by a pretty long fall. Undyne also overpowers toriel and can barely knock asgore down in training) down to
9-C against bloodlust (Flowey who is a generally bloodlusted individual cant make it past asgore without frisks help and only manages to kill them from behind while theyre at low health.)
Other stuff includes photoshop flowey dropping dozens of bombs on frisk and may also be higher with size, and mettaton neo exploding, completely destroying their body unlike in a neutral run and a highly determined geno route frisk being in front of that which i
mention in the comments would be high wall level. Though im not sure if this would actually scale to anyone else if acceptable. And sans gaster blasters are energy blasts, im not sure if they can be higher.