When it roots itself into the ground, it's going to shoot roots at you. Simply dodge these.
When he turns, he's about to throw a charged attack at you, dodge it.
He can also quickly throw a few attacks like the charged one at you.
Pick your moment in between the attacks and start attacking him. If you use the right combo (don't know your destiny/weapon of choice) he shouldn't be able to do anything.
Or use Reckoning mode.