I will concede on some things Gargoyle said, and say no to others.
Going with some abilities: Emperor has the faster attack speed and just a tad in combat speed, If he does land a shot, its game over for him. Mindset, Actually lean to the beast. Shooting myself in the foot, The Emperor is really, really arrogant. So much so the chance of him leading with his key win con, BFR is very slim. So if anything I can see the beast just lolnoping him and fights over. Although it is self bfr, counts as a win, he can at least run away to think of a strategy, but that is a big if. Time slow is going to be a pain for the beast, but even then his counter with just turning him into paste is a thing.
As much as I can try to wank the emperor (who is in my top 5 for favorite villains), it is a matter of who shoots first, I lean to the beast tho.
If he was even the slimest bit serious, emperor could send him to hell or heaven and technically he does exist at two places at once, but it wouldnt matter if you get one shot, fairly reliably even.
Inconclusive, lean to beast because bio hax. (Might as well count my vote for him for high diff, but assuming bloodlust is one, emperor and beast kinda just...get rid of eachother.) If I dont come back with a solid reasoning in a day, count my vote for Beast. I am going to dig for a wank lul.
Although fun fact, Final Fantasy II is the first (and so far only) game in the series where your stats level up the more you use them, similar to games like TES.