The final boss is pretty straightforward.
He starts trying to slap you with his hands horizontally, which is easily avoidable by just staying to the back of the platform, or you can jump out of the way. Afterwards he'll shoot rockets from his fingers. Using your magnetic power, catch a rocket (hold RT) and shoot it at his face (Whilst holding RT press X).
He will get knocked back on to the rear platform. Jump over and X combo him in the face a couple of times. The whole thing resets, this time he will start slapping the ground that releases a shockwave that you can jump over. Eventually he'll shoot rockets again. Fire one back at him. Combo him in the face once he's knocked back.
Lastly you'll get an orange beam of pain that charges over one of the 4 pads randomly. Just make sure you're not on the one with the beam, whilst avoiding his various slaps. Eventually he'll shoot rockets. Fire one back at him. Combo him in the face.
This last phase repeats a couple of times. It gets a bit tricky avoiding all the attacks and the beam of pain, so you'll probably eat a couple of hits, don't worry - if you've been playing along freeing Lo'a you should have 8 hearts or so. Don't fall off the pads because the green floor of death awaits you.
You'll be done before you know it.