Board Thread:Game Discussion/@comment-16815083-20131123011648/@comment-11482499-20131123162427

That's right mono - you need to participate in the takedown of the 1/100/1000th L8 boss. You won't know which boss was the 1/100/1000th until the event is over though (they're not specially labelled or anything).

So it's a way to get players to hit every single L8 they see, hoping that it's the magic one.