I strongly recommend that you earn this achievement by defeating the Warden. Of all the bosses he is the weakest and most predictable.
Here is the method I used.
Preparation
Speak to the thirsty prisoner give him a drink you receive a gem.
Complete Khetis side quest and collect the second gem.
Assign 1 gem to light weapons and 1 to heavy weapons.
Just before the battle speak to a caged prisoner to receive the Ring of Sobek. Equip immediately.
Unequip all of your armor. Everything other than the Ring and sword should be unequipped This way you are maximising your stamina. Equip the Bronze sword. While not the most powerful it is the most efficient.
The BattleStay as close to him as possible. This negates his long range power attack.
Keep in the centre of the screen. Do not get cornered.
Use the dodge roll technique. Instead of rolling at the last moment roll as soon as you see his arm begin to rise.
Learn his moves. There are only 3 attacks which is why he is perfect. Staying close cancels his special. However that leaves 2 main attacks. 1 is a ground pound. Simply roll past him then double attack when his sword is stuck. The second is a two stroke attack again roll past him then immediately jump back. Then hit him with a double combo. This is the tricky one as he can sometimes knick you.
Never be greedy. The bronze sword will give you enough stamina to attack 2to3times and leave enough too roll away After.
Always use RB to attackIf you are struggling try changing to the Mittani sword. This is a heavy weapon. It will inflict more damage but leaves very little stamina left. Personally I recommend the Bronze sword.
The best advice i can give is to unequip all armour, roll as soon as possible. Stay central and stay close. Allow your stamina to recharge before you strike. Never use all stamina its not worth the risk.
Here is a video.
When he is down to his final bit of health equip the Torch and finish him off This unlocks another achievement.