directive 7 - can't attack cores

Ok I am trying to defeat the Mentor at the end of the Directive 7 quest.  Solo in Story mode.

I start the battle 

I destroy the two big torrents.  The giant android appears.

I run around looking for the core with the two mini turrets ( I see no indication in the radar or map which core is vulnerable )

The cores are the spinning green things.  

I can not attack the core - no matter how close I am it says I am too far away.

Even after the two mini turret are destroyed, I am unable to attack the core  - all I can get is you are too far away.

One in a great while , for no reason I can figure out, I finally attack the core and it generally gets destroyed with one hit.

Sometimes I never can destroy the first core because the character just will not fire a weapon at the core - it is always to far away even when I am right on top of it

and trying every attack I have - nothing fires.  I click all over the damm core and 99% of the time nothing.

Sometimes I can get the second, and once I even made it to the 3rd core, but I still don't know what allows me to attack a core and when I can't (99% of the time)

I need to know how to attack the cores.  I click on them.  A tile box opens saying "core 1,2 3 or 4" but as I repeated over and over, no attack happens.

How do I attack the cores!