Shoot the legs. When the armour rips off he will expose a red panel behind his head on his neck, shoot that for a massive crit bonus. A good strategy is as you first come out of the door to the "arena" look up to your left and you should see some pipes. Jump on them and back up to the door. From here equip a sniper or scout rifle, happy hunting. P.s ignore the assholes making shity comments they forget they where in your shoes once