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.
There are several reasons why I think Kira loses.
1st for the Death Note to work one needs to know the real name of one's target (which in this case isn't Lelouch Lamperouge) and I don't think that Kira will be able to figure it out.
2nd he also needs to know the person's face which Lelouch usually covers with a mask.
3rd imo they are about equally smart.
4th Light categorically declines to make use of shinigami eyes I don't know why this is brought up as possible win condition.
5th Lelouch only needs eye contact for his Geass (= mind control) to work.
That is why I believe Lelouch wins.