The Superhero Database Classification number, or SHDB Class, is a number that represents the overall 'power' of a character. All traits of a character are used for calculating the Classification.
What it DOESN'T mean
This doesn't mean that a higher class would always beat a lower class character. But the bigger the difference in Class is, the more obvious it is who'll win in a fight.
How is this calculated
( INT^1.3 + (STR*0.5 )^2 + (SPE*0.5)^2 + DUR^1.6 + (POW + (SPS*SPL))^2 + COM^1.8 ) ^ TIER
Super Power Score and Level
Every Super Power has a score (SPS) that is used to calculate the Class. Each Super Power also has 3 levels (SPL). The level is set when connecting that Super Power to a character. The level determines the final score, of the Super Power, being used in the calculation.
The only thing that can ever kill Korosensei are those anti-me weapons. Aside from that, he is practically invulnerable and can regenerate damaged body tissues. For Superman on the other hand, while Kryptonite and Magic and Bruce Wayne are his biggest weakness, he is susceptible to injuries from anything else that uses enough force, and unlike Korosensei, he cannot regenerate. I think Korosensei takes the win, mostly because characters with only one weakness are not compatible with many other characters.