This boss is at the end of world 4-1. To do damage to the boss you need to hit its head where the bird is.
Basically there are two ways to approach the boss. Snipe the bird with a ranged attack or make it fall over so you can melee it on the ground. The first option is better because it will get you the "One Shall Stand" trophy as well.
1st strategy: Use a bow and arrow or a ranged magic ability to hit the bird. You can lock onto it and snipe it from the top level. Just make sure to dodge it's tongue attacks and try not to fall down. It's as simple as that.
2nd strategy: Run down to the bottom level where the boss is. Attack the giant piece of metal hanging out of its belly until it falls over. Then hit the bird when its on the ground.