Zeta Zeb against raid boss

The same as any other % ability works on them - minimal damage.

Ah, I was wondering if that would be a good return on investment for raids. Perhaps not.

FYI Rex does % damage with his Subdue ability, and Expose and Deathmark status effects trigger % damage as well.
For Zeta candidate, DV Leader, Thrawn Unique, Cody Leader and Finn Leader seem to be used frequently in raids IMO. Check the forum for more info.