To find the Strongman Cooler Key in Dead Island 2, travel to the northern part of Venice Beach.
Once there, you’ll find Jake Jones.
Take him out then retrieve the Strongman Cooler Key from where he falls. Now that you have the key, make your way to the strongman competition site in the northwest.
Once there, you’ll find the West Coast Weights Cooler. Unlock it to receive the Impactor Mace.