Blades of Time

Gateguard is the first boss you will encounter and is an unavoidable part of the story path. He has two uh...devices on his shoulders that shoot fireballs at you, and you need to destroy both of them using jumping attacks (lock onto one of them with LT, then dash to it with RT and then mash X). After that, use jumping attacks to damage him until he slams his head into the ground and breaks in half. His core will be vulnerable at this point and you can attack it for about 5-10 seconds before you will likely have to make him slam his head into the ground again to finish him.

I have included a video of how I beat the boss for reference. The video is from hard difficulty, but his patterns are the same on normal difficulty, he simply does less damage and possibly has a bit less health.