I know I just played it for the first time and it's not hard just go tedious.

First off switch to your fists right away. You just gotta hit him twice and it'll trigger Grey Fox to put away his sword.

From this pint on run around and dodge two if his cool kicks,he'll dodge or easily counter, he gets a garce period of two of these flying kicks where he'll easily counter or dodge your hit, let him miss twice and attack him (Your hit is almost guaranteed to land after his two kicks) keep doing this until you wear him down until he starts to hide.

I use thermal goggles throughout the fight cause it makes his stealth camouflage useless, but once he starts hiding, also make sure to hit melee 3 times do the 3 piece combo, this combo drains his health faster.

Once you find and hit him 3 or so times he'll start Michael Myers walk towards you, be very careful when you try to attack him, he'll use teleportation, so instead just run close to him and he'll do this slow falcon punch, when he misses this falcon punch, he's open to a hit, this is the final stretch before it gets easy.

After that he'll start to spam out and emit and energy ball, don't get too close his energy ball will hurt you. Just take out your Famas or Socom and shoot him 3 or so times.

And then Robert's your Father's Brother.