If someone asked 'Nathan' what was the strongest technique he learned while undergoing the 'hell'. His answer would be…
None
Depending on the compatibility with the enemy, techniques become useless or useful. There was no absolute technique to call it strongest.
But if he had to answer nonetheless, it would be 'Information'.
Master of Hell, although cruel, gave him a lot of necessary information he was missing. The reason behind his reincarnation, the truth about the world, the real enemies, and a lot more.
It was something 'Nathan' was almost thankful for. Almost.
What was the point of everything if his memories were sealed?!
"And why aren't you dying, you fatty snake!"
BOOOOOM-!
Another deep crater was carved onto the earth but Jörmungandr came out alive. 'Nathan's' attacks were not weak, rather they were too powerful, the problem lay with the [Traits] of the monster.