The way I look at it, theres really only one option; Golbez. He imperils, which everyone else except Porom does. He has magic damage, which also fits the rest of the synergy characters. And hes the only main villian/hero that fits both that hasnt been super synergy yet. The other two mages, Ultimecia and Emperor, are way too recent to dish out their bursts just yet.
Operation "This Will Most Likely End Badly" is a go.