Salt and Sanctuary

Tree of Men has three phases and the environment will play a significant role in your strategy.

Environment
All falls off the platform will result in your death. Damage from contact or fire will cause some damage but not significant; if you're on the bottom platform (where the Tree of Men is standing) try to stay centered and time your jumps to avoid being stomped but if you get hit, you usually will fly far.

The platforms around the Tree of Men are asymetrical: the ones on the right have temp platforms top and bottom, with two permanent platforms in the middle. On the left you have three permanent platforms above a single temporary platform. All permanent platforms have fire hoses to the side.

First phase
In this phase you have do enough damage to about 10 of the men hanging from the Tree of Men to kill them. You can tell which ones because they are lighted. During this phase you will have to dodge knives thrown by the Tree of Men men, a blast from the head of the Tree of Men, and (of course) the fire hoses.

To get all the men you will have to have a ranged weapon or use the platforms on both sides. I had to give a jumping attack to get a final guy out of reach from the platform.

Second phase
Once all the men are dead, the Tree of Men will be at half health and attack the platforms with a smashing blow. The timing is easy to get down (he pauses slightly before targeting the platform that you're on at that point) and you can jump away safely. The trick here is avoiding fire damage and getting to the platform ABOVE where you were standing so you can land a blow on his head.

Staying on the left side gives more permanent platforms to plan your strategies and being to the end of the platform will allow you to land two hits to the head.

Third phase
Once the Tree of Men is at about a quarter health, he starts a new attack pattern that includes breathing a wall of flame. The flame sweeps the topmost platform and then washes all lower platforms on the side you had been on.

After landing a hit you should drop down and run to the far side. Once there either wait out the fire breathing and run back or quickly ascend the far side and setup to deal damage from the opposite side.

Fire
There's so much fire going on and around the safest bet is to equip the Phoenix Rondache: a shield with 100% fire protection. While holding the shield in a guard position, you will be completely protected from fire damage regardless which direction you're hit from. Additionally, the shield has about 70% hit protection.

Cheese strategy
After the first phase, equip the Phoenix Rondache shield, ascend to the top left platform, and camp. Position yourself on the lip of the platform and whenever fire or a hit is coming, raise the shield. If fire is pouring on you, just wait. Once a hit is completed and fire is off, strike at the head. This will work for the final two phases and you only have to drink a healing potion once your health is low.

Because blocking fire or a hit will drain your stamina and leave you susceptible to damage (from either fire or a hit), it's recommended to have the shield down unless needed (to regain stamina quickly) and, if fire isn't coming when a hit is, jump above the hit ... that way you should have plenty of stamina to punish the head when it's closer.