Firstly, you need the Cardinal animal orb to get the boomerang, the cardinal is found by playing through the industrial castle level.
Just before the boss of the industrial castle you need to defeat the beefy brute enemy who has the cardinal along side him

Once you killed the beefy brute, the Cardinal will be free to take, now that you have the Cardinal pet take it to thieves forest. continue through thieves forest until you reach an owl pooping off a tree branch

As you approach the owl, your Cardinal will fly over to the bush beneath the owl and bring back the boomerang for you.
Congratulations! you now have the boomerang.
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