You can defeat Rolan in the first battle (even if there's no health bar), but he's unkillable in the second (Arma interrupts that one though, so nothing bad happens). If you aren't strong enough to beat him already, you're kind of screwed because you don't really have time to train yourself up while in the prison.
The black knight cannot be defeated in the first try, but you can beat him after he knocks you out provided you're strong enough. He just provides a different way to exit the prison, though it's better to defeat him anyway for plot reasons once you find out who he is. You need to make it to the end of the game and lose to Arma to see the bad ending to find out who he is, and then start another game which will now allow you to get the true ending
The quickest way to make Mist stronger is in the casino. Buy some coins and play the Rock-Paper-Scissors game. Get a lot of the Power Up items to buff up his strength. Once you've got about 100-150 STR, go to the basement floor and start fighting in the arena. If you are lucky and get easy fights, you can get a ridiculous number of coins that you can use to get more stat-up items and the special equipment from the coin exchange store, which is some of the best stuff in the whole game.
The exchange store on the right of the arena has all stat-boost items, though make sure you get the Power Up ones from the one on the first floor, since they're the same thing but much cheaper. Once Mist is strong enough, you can even kill the bosses in the arena that are supposed to be there to cut your streaks, at which point you can straight-up get trillions of coins with ease.
Not only can you max all Mist's stats (And Elisia's too), you can exchange coins for Mysterious Coins on the first floor exchange store, which you can take to the fairy shrine that's at the bottom of the desert continent (need the ship) and exchange it for the best equipment in the game, including a lantern accessory that cancels random encounters, the best sword and armours in the game and an accessory that makes you attack twice. You can only carry 99 Mysterious Coins at once, so you need two trips to get the 160 needed to get all the equipment.
