Golbez would be too similar in both appearance and skills as Exdeath. Golbez, like Exdeath, is a big blue knight that casts tier 3 elemental magic and Meteor also.

Instead, I wager that an FF4 tier could be:

1.0: Ogopogo
2.0 Plague Horror
3.0 Four Fiends (similar to the 4 mechs of Brute Justice, from Alexander)
4.0 Zemus/Zeromus.

For the Zemus fight, I would want it such that you NEED to caster/ranged LB3 a bunch of "Zeromus' Malice/Zeromus Mind" adds when he's at 5% or they wipe the raid. For Savage, this holds true around 70%. This would mimic the fact that Zeromus isn't born until Zemus is defeated with Meteor during his encounter in FF4. The Savage version could also grant you a "Crystal" that you use with Duty Action to allow you to damage him (or maybe to negate the damage from Big Bang).