And what would his stats be? How would you beat him?

For me the answer is easy.

The final boss would be:

Stalin
HP: 32,000
ATP: 122
VIT: 145
INT: 50
EXP: 20,000
GP: 5,000

The battle with Stalin is the final battle of the game, in which you must fight atop the floating city of Stalingrad. First of all, do NOT go into this fight with any characters below level 60. You will be CRUSHED if your characters are not at least 60 for this fight. 70 is recommended. Know that Stalin's Iron Fist of Communism can and will deal 4,000 points of damage to all party members. Not only that, when he uses it he also drains away a small fraction of that HP and gives it to himself. Not good!

He will also occasionaly cast stone skin and become invulnerable for a few rounds. During this time, the cold weather of Stalingrad will more than likely wear down your characters for 2,000 points of damage a round. Use this time to heal up and cast attack enhancing magic. When stone skin wears off go back to pounding away at him. He has A LOT of HP, so the fight might take a while.

And the optional bosses:

Galileo
HP: 65,535
ATP: 255
VIT: 200
INT: 255
EXP: 40,000
GP: 0


Galileo is probably the hardest boss in the game. He has a boatload of HP, and no real weaknesses. He will use the gravity based helio-centric theory every now and then to take away 75% of your characters' max hp. He can also summon Aristarchus to deal 5000-6000 points of type-less damage to one character. The only real hope you have in this battle is to silence him. It will stop him from using the dreaded helio-centric theory spell, but it won't stop his powerful physical attacks or his Aristarchus summon. Be careful, Silence does not always work!

And of course...

Rasputin
HP: 1,000,000
ATP: 1
VIT: 255
INT: 130
EXP: 500,000
GP: 3,000

Rasputin is sort of an oddity among optional bosses. He can't really kill you, but you can't really kill him either. After dealing about 50,000 points of damage to him, he'll cast Cure 3 and recover most of it. You're faced with a decision at this point. You can either keep pounding away at him and hope he runs out of MP, or you can run away. Poison also has no effect in this fight, so don't try to poison him and leave the game running for 3 hours.