Crackdown 3: Campaign

For anyone having issues with this guy like I was on Legendary difficulty -
Get to the Supply Point at the top of his Citadel.
Change your weapons here to:
Decimator
Jackhammer
(Third Weapon doesn't matter)
Gadget - Ammo Field.

Then, rather than running forward and being propelled into Ngata's Chamber, head off at 10 o' clock from the supply point, jump up some ventilation ducting and then onto the top of the building.
As you head up, you will notice that the top of the atrium is open.
From the lowest left corner (other locations may be possible, this is just where I did it from) you should be able to jump straight up, locking on to Ngata.
Let rip with all your ammo.
When low, throw the Ammo Field at your feet.
Rinse, Repeat until he's dead.

Good luck.

UPDATE
The previously mentioned methods have been patched. If you return to the Supply Point you will now be notified that you need to “Proceed to Ngata”. Once back on the roof, it will again tell you to “Eliminate Ngata” and his health will be fully restored.

On Legendary, with the amount of secondary forces and the nearly insta-kill beam, this boss is woeful unbalanced.

NEW SOLUTION
Similar to the previous solutions, but tweaked so that you do not have to return down to the Supply Point.

NOTE: My agent is Level 5 in explosives. This likely had an impact in damage per shot, so please be aware.

LOADOUT: Decimator, Jackhammer (NOT the Quackhammer, not functionally the same), and Anubis. I had no success with Arc or Incendiary Grenades and did not use them in my successful attempt.

METHOD: From the Supply Point on top of Ngata’s tower, just before the wind tunnel that will push you into the boss battle, climb directly up the incline. If you have double jump, you’ll be able to do it in a straight line from the Supply Point. If not, go to either side of the building and use the edges to climb up, aiming again directly forward from the Supply Point following the tunnel. After a single tier change on the roof, cut to the middle and notice a large open section and look down, you’ll see Ngata.

Lock on, take your time, and don’t fall down. If somehow you are under Explosive 5 and you burn through all three weapons (I had about 46 rounds left in the Anubis, my last gun), you can at least drop down, get some ammo and hit him with everything you’ve got and still be better off than as originally designed.

Good luck! Hope this helps!

Instead of going through the suction tunnel, jump above it. Continue following a series of jumps n climbs till you seen a giant hole one front of you near the roof of the building, drop into this hole but double dash and jump to the far right, there will be two small cylindrical platforms there. Jump on either one of these, slowly walk to the edge. From here you should be able to destroy him without fighting any other enemies and without him moving any. Equip explosive weapons and a grenade gadget for good measure. I used the Decimator, Jackhammer, and Homing Missile, plus the Arc Grenade.

Also a great place to get your "Dance, Fools!" achievement as well as your "Just Making Sure" achievement. If you have any questions or struggles just let me know so I can improve this guide, thanks!