How Should I Scroll my Fafnir Angelic Shooter?

Weapon multiplier and %ATT are irrelevant when comparing 8 att vs. 5 att 5 all. They are individual multiplicative components of the damage formula, and are independent of flat att and stat boosts. However, %ATT can be used to calculate your total ATT if you don't feel like adding them up manually. It's the same idea with %stats being used to calculate your total stats.

If you want to be exact, I'd compare these two values:

Scenario 1, 8 att: (ATT+10*8)*(4*((bDEX*(1+%MW)+pDEX)*%DEX+npDEX)+((bSTR*(1+%MW)+pSTR)*%STR+npSTR))
Scenario 2, 5 att 5 all: (ATT+10*5)*(4*((bDEX*(1+%MW)+pDEX+50)*%DEX+npDEX)+((bSTR*(1+%MW)+pSTR+50)*%STR+npSTR))

where the prefixes are: b = base stat, p = stat affected by potential, np = stat not affected by potential. ATT, pDEX, and pSTR should be the values with your clean AB weapon equipped.

And see which scenario gives the higher value. Of course, this assumes a +10 scrolled, unenhanced weapon. Any subsequent enhancements will most likely point toward the conclusion that 8 att is superior to 5 att 5 all, anyway.

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.

ATTACK / (1+ATK%) attack = (4*DEX+STR) / (5+4*STR%+DEX%) all stat

just need to know total atk/dex/str and %atk/dex/str

Would this be with no weapon equipped?

If so: 320 ATT, 5274 DEX, 1198 STR, 0% ATT, 165% DEX, 42% STR (196% DEX/71% STR with %All stat included).