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.
Ancalagon the Black has had near zero feats, and bugger all abilities and pwers, not to mention is mortal. Yeah hes BIG which can be both an advantage and disadvantage, mobility and agility is at risk.
Also he was killed by mortals.
Alduin, also known as The World-Eater, God of Time, and the God of Destruction, first creation of Akatosh, (the Cheif of the 9 Divines, Gods). In comparison to Ancalagon, Alduin is immortal and connot die, he has abilities/powers such as:
Magic
Immortality
Thu'um
Great strength
Great mobility
Ressurecton/Necromancy
Pyrokinesis
Necrokinesis
Excellent fluency in Dovahzul
Ability to travel to and from Sovngarde at will (Sovngarde is Nordic Afterlife/Heaven)
Control over a massive self-created fog made of souls
Managing to devour and absorb the souls of those in Sovngarde (Sovngarde is Nordic Afterlife/Heaven).
Reality/Time Warping
The ability to grow and EAT THE WORLD!
ETC.
One of his well-known abilities is to ressurect dead dragons, a dragon necromancer, he was also the Commander of the dragon army, his battle knowledge and battle IQ was developed over thousands of years.
The only way to defeat (doesn't mean get killed) Alduin is by a Dragonborn, a man with the soul of a dragon who posses the power of the Voice (Dragon Shout power).
In my opinion, it would be a slaughter, Alduin could probably just resurrect Ancalagon after hes done with him and have him be part of his dragon army.