I honestly appretiate your feedback as i am working on my own guide and this will help me to know what to focus and what to explain.
I must say that you are wrong about the mercenary, as you probably are not that experienced yet.
I dont know your level, but you should be atleast lvl 75 in act 4 hell.
About your mercenary. Go to nightmare and hire the one with holy freeze aura, preferably one on your level or very close. I dont know about your options with items and most importantly runes. Maybe you found some stuff you can use.
Weapon
Preferably something with decent dmg and life leech. Higher damage will help because dead enemy cant do dmg to your merc and life leech will help him sustain longer and survive. Even exceptional insight(runeword) weapon will be enough for killing. If you dont have eth polearm with 4 sockets, try to run few cows nightmare. If mephisto/diablo/baal nighmare to get some decent unique weapon. Also there is one decent set weapon too, set bill(hwanin). If you have some runes, you can even make obedience if you have decent weapon polearm base with 5 sockets. Merc will be beast with obedience even if it is not in elite polearm.
Armor
Usualy you will want something with resistances so merc can have 75% on hell. You can make treachery, lionheart or Smoke for him. Check them out. Some nightmare or even normal unique armors are also useful, duriels shell is great. Or anything with high def, life leech or open wounds.
Just open wounds can help your case, if that procs from merc attack, monster will keep getting dmg on its own for some time.
Helm
You can find some decent helm if you farm NM bosses few times, something with life leech would be great.
So how to deal with double immunity in your case? Let merc to get aura first, go to cold planins snd let him kill few fallens to activate his aura, then he will keep freezing monsters around him which greatly helps as monsters cant attack him so fast.
Use telekinesis to lower hp of monster to 50% and if monster is especially tough, spam telekinesis to monsters against object or wall to help your merc in his fight.
If he doesnt have great items, yes he will be dying sometimes, just play smart, dont just let him run into big packs of monsters and just tank. Use teleport to go back if he is taking too much heat and heal him.
Chaos sanctuary is especially dangerous so yes your merc will be dying there a lot, just keep healing him with potions and take it very slow.
I hope this helped.