Pulseman Tree Boss

I have a part of Pulseman that I am stuck at. It is the India level from the first level select screen. How do you damange this boss? I can't seem to damage him when he pops out of the ground, while shooting that orb at me, or when it sends it's roots after me.

If I remember it right, maybe I have to fire over him, and attack it from behind? I just don't see any week spots on him.