Go through entire level on apprentice and first 2 parts of boss.
After the floor falls through for the second time change to your desired difficulty
------LET HIM KILL YOU------
Then change back to apprentice to continue the fight (not necessary just makes it easier)
------DONT DIE AGAIN------
Once the boss is dead the achievement of the desired difficulty will be yours.
Pause the game when the achievement pops and exit.
Then choose continue game and you will be at the start of the fight and you will be in the difficulty you chose for the achievement so just change rinse and repeat.
P.S I just used lightning on him in apprentice and he went down quickly it took me all of 10 mins to get all 4 difficulty achievements
I have found out there is a hidden requirement
Credit to HolyHa1fDead :
This achievement has a hidden requirement that you must first complete the main game on any difficulty