Have you been hitting the boss with a powerful, fully charged attack? The boss from a couple weeks ago has a crit spot on the back that you have to hit with a charged attack and that may be the one you missed. Either way, make sure to hit this week's boss and the two next weeks with a powerful attack and hope that those are one of the two you're missing. To reiterate, it's not necessarily completing the event, but hitting the boss with the charged power attack. Good luck, my friend.