If you don't have anyone for local coop , just play solo untill you come across a boomerang. I did it on stage 6. Get a second controller ready , signed in on a secondary profile but wait to join the game untill you cleared the screen of enemies where the boomerang lies on the ground. When it's clear , press start on controller 2 to join and pick up the boomerang with player 2 and just line up properly with player 1 and throw it in his direction . He will catch it automatically as long as you both are lined up properly ,unlocking the achievement.