So I fought an intense duel today in Guild Wars.
I was playing Random Arenas (4 v 4), and the final 2 combatants left were an anti-caster Mesmer (me - full health) and an Assassin (some other guy - full health).
As the dust fell and the last breaths of their comrades expired, the battle weary, bloodstained Assassin, and the mysterious powerful Mesmer, approached each other wearily. A strange sense of calm descended over the battlefield. Feeling a certain nobility overcome them, the two foes bowed to each other before stalking in for the kill.
Suddenly, the assassin teleported onto the mesmer, who was promptly knocked over on the ground, interrupting his spell casting. Gleefully, the assassin sliced the mesmer into various pieces, then laughed maniacally and ran around in circles shouting "rotfl noob mesmer!!!!!~1111".
In summary, nerf assassins please.
Wait, is that the right response here? I hear a lot of people criticise various MMO's for not being balanced because class XXX has no chance in a duel against class YYY.
But guess what? MMO's, and RPG's in general, are all about the TEAM dynamic. By definition, in a Role Playing Game, we all have a different role to play. So what if some classes will almost always beat other classes in 1 on 1? The game is designed around being balanced in team form, so I'd be surprised if that WEREN'T the case.
The same gripes are encountered in PvE. Boo hoo, my healer does no damage, boo hoo, my nuker has weak armor, boo hoo, my tank can't heal. Who cares? You're all supposed to be working together anyway. If we could all do everything, we wouldn't need anyone else to play with, and for that you may as well be playing a solo RPG. Which, by the way, require the same diversity of classes (at least, the good ones do), none of which would ever be balanced in a 1v1 duel.
Of course, it's not quite as simple as all that. Real problems arise in MMO's where being ganked is a possibility, since there's little chance to defend yourself against many classes.
But is that a problem of game balance? Sounds more to me like it's a problem of there not being sufficient risk for would-be gankers. Allow things such as looting of gankers, or an honour/dishonour system, or bounties, or penalties for ganking lower levels, and these worries go away.
Then so too goes away your problem of having 1v1 battles being imbalanced, and the game developers are free to make a rich, diverse team balanced game.