Chasm Ziggurat - Oni Stalker Boss - Part 1

The Oni stalker is strong and well armored. Even penetrating arrows just ricochet off his metal plates (as shown above).

Fortunately he doesn't move all that fast. So as soon as you regain control of Lara after the cut scene, try to put some distance between you while you get your bearings. Scramble out of his way...

...and then run around the edge of the arena in either direction to keep ahead of him.

His back is his only vulnerable spot at first. The trick is to stand your ground as long as possible to lure him into taking a swing at you.

Then just as he starts to swing, dodge out of the way by pressing Dodge/Roll (Shift/Circle/B) and Left or Right simultaneously.

His club will smash into the ground and get stuck there briefly. While he struggles to lift it, get around beside or behind him, and shoot him in the back where you can see bare skin.

You'll definitely be able to tell if you're doing damage. I recommend the shotgun here, but use whichever weapon you're most comfortable with. In any case, you'll need to do this several times in order to injure him enough to move on to the next stage of the battle.