the first way i did it was by using the anvil hammer and when it kept firing wind from its mouth the barrier would be gone so you then run in and quickly bash it on the head after some time the barrier goes down you can also do this with a dragon element longsword hope this helps and good luck
Spirit Blader: An easy way to take down Kushala is to use flash bombs to temporarily stop his wind aura. When he is down, attack his head repeatedly. The Eager Cleaver or Devil Slicer (or even the Black Weapons) are good to use because none of Kushalas "parts" can be broken without Thunder or Dragon element. Once you break his horn he will no longer have a wind aura. That's when he becomes a pushover.
In short, watch out for wind blasts, flash bomb him, break his horn and youll do fine.