NOTE: played on Emerald Knight difficulty
For this Ultra Manhunter boss, you'll need to remember one thing. Blocking is a Green Lantern's best friend.
Every time the manhunter readies an attack, block.
You will be knocked back but only suffer damage if you're sucked into the gravity well of the purple spheres the manhunter deploys. And that damage is minimal even on the hardest difficulty.
Between blocks, beat the manhunter's health down with strong attacks and hammer construct blows. Those will usually take down 2% of health per hit.
If you're unlucky enough to suffer hits from the robot's attacks, there is a power battery in the back you can use to heal up.
But overall, with block, this is just a matter of whittling the giant hunk of junk down to size.