Before actually giving my thoughts on where characters scale, I want to quickly cover Clover’s Shmup mode. Shmup Clover is an indication that they’ve received a massive power boost on top of just the new abilities. For example Shmup Paci Clover’s Big Shot easily overwhelmed full power Ceroba, whereas Geno Clover barely fazed her with their attacks despite having killing intent. Paci Clover’s Big Shot also lead to Ceroba immediately going for her last resort, showing that Ceroba knew she couldn’t win without it. One bit of
flavour text in the Kitsune Ceroba fight also directly states that Clover’s Soul “pulses with great power”. In Geno, Shmup Clover’s Big Shot was able to effortlessly destroy a shield that wasn’t even scratched by their prior attacks. Both the Pacifist and Genocide Shmup modes are acquired through extreme emotions as well (Determination in Pacifist and anger in Genocide), implying they’re similar in power (not that it matters but I figured I’d mention it). The Meta Flowey Shmup mode also likely came from Determination, as Flowey kept noting Clover’s stubbornness and Clover is seemingly able to escape Flowey’s clutches everytime he depletes their HP.
Characters prior to the Steamworks should scale to “
9-B, possibly/likely
9-A” based on Dalv surviving an encounter with the Blue Soul human.
Axis and those who scale to him should be fully
9-A as he (seemingly easily) mutilated the Blue Soul human to the point that Chujin was sickened by the sight of their remains. Although it’s possible, I find it unlikely that someone as violent as the Blue Soul human didn’t encounter a monster in Waterfall prior to Axis finding them. Axis was also created specifically as a defence against humans and is capable of harming a LV12 Clover despite being portrayed as weaker than them.
As I’ve made it pretty clear, Ceroba should scale to “At least
9-A”, being bare minimum Asgore level while she’s at full power. Kitsune Ceroba scales far above her base form’s full strength, as she’s able to fight evenly with Shmup Clover.
Zenith Martlet obviously scales way above every other monster that appears in Yellow. I think “At least
9-A, likely
far higher” is appropriate considering how she can tank multiple lasers from Geno Clover, who’s Soul alone was so tough by that point that Asgore’s trident shattered upon contact.
Hopefully this is coherent and has everything I wanted to say, I had to delete and move around some stuff.