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:
(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.