How Should I Scroll my Fafnir Angelic Shooter?

Not entirely sure what you mean when you say "stat affected by potential/not affected by potential", but since I'm planning to throw a 5-Star on the weapon, I'm assuming the +8 ATT scrolls will end up being the better choice.

Stats not affected by potential include Inner Ability and Character Card (read: card, not deck) bonuses. Stats affected by potential include bonuses from everything else (equips, skills, etc.)

My method is much more tedious compared to MountLag's (well it's more or less the same concept; his is simplified so you arrive at a conclusion quicker).