If you want to leave Oogles a tip for writing this Metal Gear Solid: Rising Revengeance guide you can do so here.
Metal Gear Solid: Rising Revengeance Walkthrough Boss Battle: Senator Armstrong Part 1
Metal Gear Solid: Rising Revengeance Boss Battle: Senator Armstrong Part 1
As soon as the battle starts go on the offensive and start attacking Armstrong with the polearm. Block his attacks whenever it comes at you.
You'll notice that you're only doing a .1% damage on him with every hit that you make. That's normal, you're not doing anything wrong with the battle.
When he starts glowing yellow, run away and jump if you can to avoid damage. He has 2 unblockable attacks. 1 is the wide area attack and the second on is the grapple, which you can avoid by ninja running around, then jumping away as soon as he gets close.
I'm not sure whether you need to get his HP down to 93% or you only need to survive for a certain duration of time, but in any case the battle will be put to a halt by a cutscene.
After the cutscene you'll have to take Armstrong on without your sword. You don't really have to do anything at this point. Just stand and get yourself beaten to death.
When your HP hits 0.1% QTE will appear on the screen and you'll have to press them to damage Armstrong. Which of course won't do much damage on him either. A cutscene will play soon afterwards.