User blog comment:B214/Volume 22 spoilers/@comment-27083298-20161112215056/@comment-10330822-20161114193814

Also, we have to consider Issei could also just strengthen himself instead of just unlocking more powerful forms. After all, if Issei physically, but himself with no armor, could make himself an Ultimate-Class Devil, he'd be even stronger in Scale Mail. After all Euclid, who was an Ultimate-Class, was able to fighting Issei using Cardinal Crimson Promotion using the Boosted Gear Scale Mail Replica, despite the fact Issei should have been drawing on Ddraig's true power.