Island is to the north of the pirate island in future times.
First kill the lone chicken to summon the chicken frenzy!
I did this at level 25. Use velvet to freeze some chickens, kill them.
Repeat until you are down to a few, then let them charge you, dodge out of the way and hit them when they are stunned.
When you have them all, you will pop this achievement and will be able to take the treasure from the chest to the west.