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.
GG
Or, more generally speaking: Do not underestimate main protagonists of popular long-running shonen manga.
These characters like Natsu - they have a well-known tendency to adapt to however powerful their opponent may be (even if their opponent should objectively be way stronger than them), and then still win.
That's just who they are, their nature.
Their job / role in the story is to win - at least eventually, and even when there may be some temporary setbacks on the way until they do.
Natsu with buffs from 7 Dragon Slayers (all of which are present in this team) has also one-shotted 5-D Acnologia,, so he also has feats which suggest he could play in a league comparable to that of Dormmamu.