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.
Bard on the other hand has been around for far longer and likely has faced at least vaguely similar opponents considering for how long he has fought the Void, and even if not that he would likely just have knowledge about her considering her prominence as a Darkin and that he is the safekeeper of Runeterra. So id imagine eventho she has a pretty good shot at fighting him, Bard would ultimately win through some tricky use of his magic.