This might not be the fastest way but it enabled me to kill him on my first trip to the crypts as a caster:
First I cleared out the area to the right and below of the square “room” he is in, staying away from the entrances. Then I went in and lured him out, staying close though so he didnt run back and regen his hp.
After that I just ran in circles casting my fireballs.
If you want to go safe: keep running until he does his ranged attack (he stops moving and swings his weapon around) since he targets the spot where you stood when he started that attack you have time to shoot 2-3 fireballs at him if you move a little bit to the side.
If you dont mind a little extra risk: you can shoot a fireball off when your decently far away from him, he will probably catch up to you and start his attack, but since it takes a bit to charge up you evade it if you move immediately after shooting off that one fireball.