User blog comment:B214/Volume 21 info & spoilers/@comment-191.82.149.185-20160704214534/@comment-24.113.252.213-20160707042221

Because instead of spamming [Boost], he's using Illegal Move Triaina, and Cardinal Crimson Promotion.

These forms enhance Issei's strength by a flat amount, reducing his need to use [Boost].

So even if he's only using [Boost] 10 to 20 times in his fights, it's still:

Issei (base) + Scail Mail (whichever form he's using at the time) x 2^n (n = # of [Boost]s)

Issei is still enhancing his strength to several hundred/thousand times his base strength.