This event is bugged and it won't complete, nor will it reset on its own (it's been hours since the High Sage was killed).
The only way for this event to reset is if Lake Doric fills up with enough people to start a new IP instance, which is unlikely because of the low popularity of this map, or if Anet updates the game with a bug fix or new patch (which could take months) and wipes this IP starting a new instance once the game has been patched.
The people going for the "Those Giant Hands" achievement along with the mastery point are not able to progress on their account due to this bug.
Again; it may take months for this to be "fixed" with a patch/update, which seems unfair for those who are going for the achievement and mastery point.
Attached is an image of the bug.