The Guardian wields a large ball and chain that it can twirl around and knock you away with so avoid getting close to it. Around this arena you'll find various orange portal stations on the ground. Shoot a charged portal at one in the corner then another into a nearby portal. At this point, lure the Guardian towards the one in the corner. It will eventually pull back and then swing the ball and chain down at you. Dash out of the way then hop into the portal and then use Shadowflight to float down on top of the Guardian (remember to use charged up portals).
Once you land on the Guardian's head, press the Action button to stab your Chaoseater into its head. This will stun the Guardian and cause it to collapse. Jump down and then perform a quick combo to its glowing torso. Once it turns red, Dash away from it to avoid it's twirling attack which can be extremely damaging. Once it calms down, create two more portals like before and again, lure it near one only to use the portal to fling yourself onto its head and stab once again. You'll need to stab it then combo the torso three times. During the third attack, watch for the action button to appear so you can finish the Guardian off.
