I like the idea but like others said... it'd be really hard to guarantee a loss. They could maybe block trading until after the encounter but it still would be possible to train forever (almost literally forever...) and get your pokemon high enough. I guess they could modify the experience system to give 0xp when you get a certain amount of levels ahead (maybe just wild pokemon?). Also I don't know if people would be happy having to wait to trade.
I guess the easiest way would be to do it very early on... but then it's a boring fight with probably just your starter. Too early and you're stuck with a super small pool of pokemon, too late and it's harder to control what you'll have in your party and still guarantee a loss. Plus you need a nice amount of time for you to really work up to defeating them.
Also I'm not sure Pokemon would lend itself too well to this type of situation... It would just be the opponent instantly disposing of your party. You would just have to knowingly send your pokemon to their doom...
That all being said, if they can make it work I'd love to see it. Also I don't think I've played that many games that have a situation like this (not really a fan of JRPGs) so I don't know what kind of situations other games have.
This user is awesome!: http://boards.gamefaqs.com/gfaqs/user.php?board=927246&topic=44002798
I guess it depends on how they pull it off and the context in which you have to fight him/her. Like if they have ridiculously high leveled pokemon compared to yours. I'm kinda uneasy about the idea, but it could be cool if they get it right.
Playing) Fire Emblem: Awakening, Guild Wars 2 Waiting) Pokemon XY
Many games have done this over the years; introduce a new incredibly strong character early in the game for you fight. You're too weak for him and he takes the win. This gives you encouragement to get stronger in order to beat him in your next confrontation.
Some Pokemon games tried to do with e.g Norman, Aqua/Magma Leader and Fantina(I think). But it wasn't the same, as you never even got to battle them.
So what do you guys think, should there be an unbeatable foe early in the game for you to battle?