User blog comment:Piyush Banerjee/Shin DxD 2 discussion/@comment-2602:306:CD9A:C860:1896:46C0:1D50:41A4-20181226150049/@comment-33939194-20181228212209

Actually Rias was more challenging than Kiba. Although Kiba hit him multiple times. He did little damage to his armor. The only problem Issei had was his speed. Against Rias, Issei had very limited options to work with. His dressbreak and Pailigual was useless. Close combat was out of the question due to Rias could just teleport through the shadow. Longinus Smasher and Infinity Blaster was useless because Rias could just avoid it with teleport. Unlike Kiba, Rias could seriously damage Issei's armor with her POD attacks which is why he had to cancel them out with his own attacks and on top of that he had to stay out of her line of eye due to time skip.

Rias Balor form pretty much nullified 99% of Issei's options to deal with her. He literally had to catch her off guard with a move she didn't expect. It's not just Rias was stronger but more troublesome to deal with.