in
is actually winnable. I brought all party members to level 99, maxed out Enah's MND stat (999), upgraded the Energy damage bonus (Aid Room) to maximum (100%) and used the "Double Damage" pay-to-shortcut DLC. Enah was close to one-shotting him, so I was actually crazy-prepared; nevertheless, the opponent can still one-shot every party member at this point, so this fight needs to be taken seriously from beginning to end. Edit: Alphadia I features another "unwinnable" boss fight against the exact same opponent (with the exact same stats) at a later point, though if you could manage to "win" the first battle, you can and will "win" the second battle, too.
As for the reward: There's none, since the player isn't supposed to win. At least the game didn't crash; the game crashing is something that happens in many (especially RPG Maker) indie JRPGs when the player wins a boss fight that is really supposed to be unwinnable.
The very first fight of Alphadia I remains unwinnable (under normal conditions), though, since there isn't any opportunity to get stronger beforehand. The only possibility I can think of is to obtain the necessary offensive Aid Room upgrades beforehand, but I think the damage output still wouldn't be high enough, while the opponent can one-shot the only two party members every tun (that means he would have to miss his attacks a few turns in a row, which is very unlikely to happen, if it's even possible at all). Sometimes you just can't win (at first).