Robin Hood - Sherwood Builders

Defeat the boss (Bow only) during the Midlands quest - On Earth, as It Is Up There.

The quest starting point can be found within Nottingham, at the St. Mary's Church Mysterious Location map marker. Speak to the Friar inside to initiate the quest. Fight the attackers to receive the key to the door behind the altar. Head up the stairs and take note of the 3 angel statues to solve the puzzle in the next room, unlocking a hidden stairway into the catacombs. Fight through the enemies below and activate the 2 larger angel statues to raise the gates and access a lift down. Reaching the large doors will create an autosave checkpoint, luckily, as passing through will begin the boss fight, with Guy of Gisbourne.

This is a difficult battle, Guy tosses poison vials, can only be hit with arrows, and the arena slowly fills with water. Even if you kill him, you still have to platform across the area to escape, there are no checkpoints along the way. When standing in the water, there is a point when the it is high enough that prevents you from jumping, making it impossible to continue. You will need to find a balance between getting to higher ground, dodging poison or consuming an antidote, and then firing off arrows. Dying anytime prior to defeating Guy, exiting the arena, and reaching a checkpoint or saving at a bed starts the entire battle over, to say the least this is extremely annoying.

I found it best to start by trying to knock off his helmet and then continuing to try and hit him in the head for critical hits. I was often able to kill him on the first platform with several well placed headshots, but most often discovered the water had risen too high and I was unable to reach the next platform to continue. Use Slow motion and reduce the aim sensitivity if you are having trouble. If you work quickly, you should be able to easily dwindle his heath down to 33% or lower, but your goal should be at least 50% before moving on. As you start to move up, he will also move to higher platforms and the distance of your shots will also increase.

Once Guy is defeated, the achievement pops... escaping the arena is another matter, good luck!

This is a very tricky boss battle. You can't fight him with a sword, you can only damage him with arrows or projectiles. He is in the middle of the arena and you're on the outside. He fires arrows and poison bombs which drains your health and obscures your vision.
You have to climb around the edges of the arena on platforms that come out of the wall, all the while he's throwing arrows and bombs at you. Not to mention there's rising water and if you go too far under or fall off a platform it's instant death!
Id recommend you get special arrows such as the fire/explosive arrows because they do a lot of damage to him. Also make sure you have plenty of healing items and antidotes for the battle because you'll definitely need them.
Once you beat him there's a few parkour jumps before reaching the top, if you fall you'll have to do the boss all over again.
Once you're out of the arena GO TO A PLACE WHERE YOU CAN SAVE OR GET A CHECKPOINT! I made this mistake and died and had to do the boss again...
This is a very annoying boss but good luck!