Hi,

it took me ages too.

In the end I used the riffle and all the rounds I could get from the henchmen in the boardroom shootout – about 120ish rounds.

Then I sniped away at him from the walkway ducking back behind the rocks to avoid the energy wave things.

That takes his health point down to about 25%.

Then take him out with pistols and grenades while jumping about a bit and using health packs – I got through all three.

As I say, it took me a while to get right, but take all the ammo and health packs you can.

Hope this helps.