General

Prime scrolls for Xenon, always, no question. iCoG/CoG for anything else if you're confident you'll get good stats (nothing less than +2 ATT per scroll and no fails) I'd say.

Well the prime scrolls at least offer a guaranteed 90+ stat which coupled with your own % luk is usually 2~3x that.
Like the guy mentioned above xenon ofc is the best to use those scrolls for, icogs and cogs are pretty sketchy sometimes
(like wasting a slot and getting +0 attack) or just using icogs and getting +1 attack meh~

Do ICoGs + return scrolls and aim for +6 all stat +6 att per scroll

iCog, if you can, is always the best;
and you may get + att/m.att (better than your mainstat)
Unless you're a xenon, that is.