Now just add a cool idea how to distinguish between real players and fake accounts added just for the reward....
Well, there could be several options:
* Every member in the party has to actively participate at least in killing the final boss. To avoid just "being carried", maybe this would help to avoid players making a char in a second computer that would only follow and stay at a safe distance.
* Only works in full party. This also prevents using a second computer with fake accounts, unless you have 4 computers and actually can make them participate in the dungeon run (read first point)
* It could be a not-so-super attractive reward. If it's only a temporal exp buff it wouldn't be as game breaking as a stats buff, yet still attractive enough. So you wouldn't mind even if someone actually made fake accounts to get a slight buff in their grinds.