u/RuinSimilar7798

From what I understand, Diablo needed "Slime Cells + TN" to rival Feldway's power; he won due to a difference in skill, mastery, and combat sense.

So, since the events of Vol23, Feldway has received Slime cells from Rimuru, and if, in addition to that, Rimuru allows him to exploit TN....

Diablo (Slime Cells + TN) >= Feldway

Feldway (Slime Cells + TN) > Diablo

Therefore, Diablo's only way to close the gap is to possess a Genesis-grade weapon to increase his chances of victory, As was the case for Guy against Velzard but that's something that can never happen in the story, because the main story is over.