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You pick the set that has your favorite set bonus, then you craft the units. Really don't bother with their base stats, because they were kinda weak when they came out, and they're even more nowadays.

Yellow thunderbolts sounds like the Bind status, which reduces ennemy movement and can only be done through skills such as RA's Bind Bullet or FI's Chase Bind.
I'm guessing you had NPCs in your...

BRFI doesn't have much going for it, but FIBR has limit break shenanigans.
As you said, HU is the go-to sub.
BRRA is strong for bows, and BRGU may get you banned due to chain combo + banish arrow...

Break the rings to open the weak spot, start chain combo with weak bullet + homing emission PA abuse. The character spins on itself to lock on more targets (ringahda body parts), and the more targets...