Not too long into the game, you'll come across your big free-roam area, Croakwood, just after leaving the village, Markovna. In this area, you'll find an ominous scarecrow in the far top right, picture below for exact location.
Once you interact with said scarecrow, you'll teleport into another realm. In here, there'll be a scarecrow boss, with an infinite spawn of crow mob. Just blast through, or run around the crows and hit at the scarecrow, who has quite a bit of low HP. About halfway down, he'll get a shield that'll make him invincible for a short time. Use this time to heal up, or do whatever you need to do if you're low on your various meters. Once the shield disappears automatically, go ahead and finishing him off, for a trophy and some loot.