Are the "power ups" from the ultimate weapons added to the attack or do they decide the entire amount of damage?
The damage from ultimate weapons such as the Captain's Venus Gospel would become worse as the battle progresses if it is the latter since the power up is increased as the MP amount is closer to the maximum MP amount (assuming that magic is used but MP isn't replenished).
I know that at least Mr Valentine's Death Penalty could be crappier than regular weapons but it differs from other ultimate weapons as it includes several factors such as level and enemies killed.
PS. It has nothing to do with the subject of this thread but can you see Nanaki's weapons in battle?