As far as I know every class performs poorly against him, best way to deal with malthael is a combination of high dps, outsustain and abilities that provides immunity against inability effects so you need to kill him fast, regenerate fast, have enough life to avoid being one shotted and don't get stunned/frozen whatever. that's what I do and works very well.
Btw never had problems with him on my Arachyr Wd, he just dies way too fast, prolly you need better gear
swap convention of elements for unity in cube and set unity in follower it will up your survivability by a lot
spirit vessel should be your 5th passive incase of hellfire amulet
if you need extra survival swap Boon of the hoarder for Molten and if still needs to survive even more trade bane of power for esoteric