You should use arrows to attack General which you have in your equipment. The biggest damage will deal Blast Arrow, but you can use also other types, like Fire Arrow. Each time remember that drawing a bow takes several seconds, so don't stand in the open space while preparing to the shot. I recommend standing always behind a pillar and lean out to fire and then hide again to avoid a General's counterattack.
If you are OK with reveling your position, and if you have unlocked the upgrades to your Focus, connected with combat, you may try running up to the boss, turning Focus on and cripple one of his lower limbs (this requires you to hold down an appropriate key/button) by attacking it with the blakckjack. This attack will open him for more strikes. You can also use the moment when General is stunned and quickly pick up two new Blast Arrows from the chest on the table.
Continue attacking boss. His health bar should be displayed over his head. Remember to move within the outer part of the arena and hiding behind pillars, so he can't reach you. When you almost zero the General's health bar, he should kneel. When it happens, you have to approach him and hold the button marked by the game to finish him off.
Now you just need to use the two valves and lockpicks, what is discussed on the previous page. As you don't have to avoid the boss, unlocking the exit will be a piece of cake.