Even though Thief only has two boss fights, both are a challenge. For the Thief-Taker General, there are two approaches to winning this battle. The first and most common approach is to damage to your foe, dropping him to one knee when you've won. The second and more difficult way to emerge victorious is by not harming him at all, making your escape using stealth and timing.
Learn more Thief strategies with Prima's eGuide.
The Lethal Approach
When the cutscene ends, you'll find yourself on the outer perimeter of the room. Take cover behind one of the pillars, swooping counterclockwise -- to your right -- to avoid being spotted or attacked. Try using Water Arrows to douse the torches hanging from the walls. This allows you to work in the shadows. Just be aware that the Thief Taker General can light those torches again when he attacks you.
When you have an angle on the General, attack him with Blast Arrows. You can go for head shots, but we would advise aiming center mass. This ensures that your arrows hit their target, rather than being wasted if he moves at the last second. Fire one arrow, then swoop to your right and repeat the process. When he drops to one knee, the Thief-Taker General is defeated. Be sure to pick his pockets before deciding if he gets to live or die. The General carries one of the game's Collectible Loot Items, Serendi Stone Circles (7/7) - Amber Circle.
If you don't have any Blast Arrows, the concept stays the same, except you will have to use Broadhead Arrows, Fire Arrows and Sawtooth Arrows. Choke Arrows can also be used to briefly stun the General, allowing you to move in for some melee attacks.
With the battle complete and the room looted, turn the valves on each side of the room to remove the gate covering the door. Approach the door and pick the lock to finish the chapter.
The Non-Lethal Approach
This approach is somewhat more difficult to complete, yet very rewarding. Instead of damaging the Thief-Taker General, just make sure you're hidden in the shadows. Water Arrows are still your best friends. Use them to create darkness. When the coast is clear, approach the valve closest to where the battle began and turn it as far as you can.
Make your way to the other side of the room, swooping from cover to cover. If the Thief-Taker General is close by, try hitting him with a Flash Bomb to buy some time. The key here is patience. Wait for him to move around the room, turning the second valve when the opportunity presents itself.
With both valves turned and the gate no longer covering the door, move to pick the lock. It's not an easy one to conquer, so upgrading your Lockpick Quality from the Shady Merchant is ideal. You can also use a Focus point to upgrade your Dexterity, making lock picking much easier. When you pick the lock, you will leave the Thief-Taker General behind you, ending the chapter.