From in game:
Players who achieve the highest Global Kill amount during the Event Period will be declared victors and win prizes for their efforts.
Prizes must be redeemed in the Event Center within 24 hours after the Event ends. Tap on a Top Ranking Players Name to see the prize for that Rank within the Event Center!
Please, tap on the Event center icon on the left side of your screen, then open the Goblin Kill event. The objective of this event is to kill as many Goblins as possible.
In order to do that, go to Quests - Campaign Mode and select the highest level you have unlocked (for example: Goblin Infiltration). Attack the Boss on that level to get the highest kills possible.
You might want to select your best hero and troops and use any runes you have to minimize your troop loss during the attack.
Try to win and get as many kills as possible.
Please note that for the Alliance GK, if you leave your alliance, you won’t get the rewards at the end of the event.