nice... My WD die non stop at T6 Malthael . I kill him mostly for reaper wraps crafting material and found drop rate is good at T3-T4..

oh yeah, that too 
actually i can farm maghda for cain's mat quite ez in t6 (yes using unity combo).'

But malthael is a bit harder 

well actually i would think it would because your follower with the token can't die, takes zero damage. but he still helps mitigate 25% dmg for you 

I think someone even mentioned if you stack 2 unity rings it can mitigate 50%, but i'm not sure if that trick still works or not 

well... i think while he wears that token (cannot die), he takes zero damage from all sources.

Which is most likely the case, because watch your follower tank and you take zero dmg due to the unity ring.

So all dmg your taking goes to follower, but he soaks it up and not take dmg, or even cause you to take any dmg from his side.
So that is how that mechanic works on the follower :} from my observation.
Theres actually a redits article that covers a lot of info for followers (e.g. what mechanics work or don't work on followers etc...), which i recommend you check out here
http://www.reddit.com/r/Diablo/comments/24...r_ashearas_set/

Interesting topic , i did it with my noob crusader 

he is using...
The furnace 2hander mace
http://us.battle.net/d3/en/item/the-furnace

aranoch amulet = immune to cold attacks

full akkhan set

reaper wraps

harrington waistguard
There isn't much to click here to make use of the harrington. So i would think a witchin hour would be more appropriate.

The furnace is definitely strong, but i've only seen people switch to it after using dots. So maybe if you used a proper firestarter weapon like a darklight, and employ a swap strategy to furnace, wouldn't the damage output be higher ?