User blog comment:B214/Volume 22 spoilers/@comment-201.215.165.7-20170112030400/@comment-87.8.65.225-20170112094720

if you speaking about intelligence, you're right, he is a stupid (even if he has some good ideas, generally speaking is not a smart one), but in terms of resolution, Issei is absolutely strong, he is willing to face any kind of pain to protect his comrades. He gave his arms to Ddraig to face Riser; he endured an enormous pain and a reduction of his lifespan to get Vali's power, etc..