From what I can see, I can make a Fortitude (for my merc?) or Grief for my Barbarian maybe…
I cannot make any other runeword requiring a Lo, as they additionally require other HRs.
For Fortitude, obviously the 300% ED applies only to melee classes. I already have ethereal Duress and Gloom. Maybe Fortitude is not needed? Anyway at level 85, my Act 2 Merc often dies at Pindleskin with ethereal Gloom/Duress, Insight Ethereal Cryptic Axe and Vamrire Gaze. Is Fortitude better at keeping him alive?
Kiras w um for max Res or treachery for dps + fade
Better weapon, eth obedience or repears
I would go with eth treachery at this time until you get the IAS boost for him with an andies vissage, but yes a fort does help him kill things faster, which helps him live more in the end.
Definitely make grief and not fortitude.
And if I make Grief, last I played my Barbarian he is into Polearms and Concentration. So I will have to respec him to dual wield swords/axes maybe, so then he has two runewords buffs with dual wield? Or a single 5 socket two handed sword? And do I stay with Concentration (which I like)? I do not like Frenzy it is too quick.
Fortitude on merc is a noob trap (cuz omg more sheet dmg and armor) and treachery outperforms it in almost every scenario
Either make a fort for your own playable character (bowazon), make a grief for a MF barb, or trade it for something else
Here is too much information on fort vs treachery for a merc if you want:
https://forums.d2jsp.org/topic.php?t=54233648
I can buy a fort if I want for my merc no prob, but I still use Andy’s with 15% IAS, treachery, and insight Giant Thresher on my cold sorc. If you want to spend more on something you can get a 15 IAS jewel with fire res but when fade procs on your merc you’re at max fire resist so it really doesn’t matter unless you get lower resist casted on your merc / conviction.
If you go barb, this is a good guide for berserk and find item:
Some time ago I abandoned my level 80 Conc Barbarian for my level 50 Sorceress who was stuck without respecs. Eventually I got a token, respecced, and now my sorcy is 86 and does Pindle, Eldritch, Shenk and Thresh and Hell Countess with relative ease and speed and 250% MF. Why would I want to play the Barbarian again, I wonder…
I started levelling a Trapsin, for the novelty of it, but now Act 4, Hell, she is finding it hard, the party seems to be over. She literally steamrolled enemies with Wake of Fire and then with some Lightning traps, but now, Act 4 Hell, not so easy anymore. I think my sorcy at same level was having an easier time.
PS: back to the subject, I am very scared to use my Lo. I only have one of. Never will I likely see one again. Grief or Fortitude? And if Grief, on what weapon? One or two handed? I will do some simulations on maxroll…
Fort helps ranged amazon too, not just melee
Running some sims for the Concentration Barbarian, there is so much mid-tier choice (relatively CHEAP) for swords and polearms:
Unbending Will
Passion
Oath
Crescent Moon
Obedience
Insight
And Grief being the most expensive by far AND doing double damage it seems.
Still running sims…
Fortitude is always a great investment for me. After that the mercenary can take care of immunity problems easy. Can take him along Meph runs (so you get insight) because you don’t need to moat trick anymore, just face smash him and merc survives.
It’s just so handy on multiple different levels.
Use the Grief for it’s true purpose.
Bashing(smiting) Ubers with a shield. 
All jokes aside, I’d prob go with a Fort and make a Bowazon or a Throw Barb. Grief is only good for killing Ubers really… and you don’t need it to kill them sooooo.
Also for what it’s worth from what I’ve seen of the new throw barbs, they seem pretty legit. The grim Ward changes are pretty nuts.
I second eveey other person here. Grief is much more important, a necessity. Fortitude is a luxury. Guilli+Treachery+Reapers is the budget combo to keep the merc alive. If you hire holy freeze merc your merc can slow down all the mobs into slowmotion wit holy freeze and decrepify from reaper’s together together.
I finally made grief for my ubergetup change as hammerdin, for killing the freakin witches is much safer and faster with grief. My hammer hired an act5 merc. When I am not Ubering, I give him my grief and azure from prior. He’s got a crap arreat’s with 3%ll and gladiator’s bane. With that and the damage from my concentration aura he never dies.
The thing is though… Grief is only going to see use if you are actually going to “farm” with a melee build. AFAIK the only “good” farming builds that use Grief are Dream Pallies, and Trav Farming Zerk barbs. One is super expensive, the other is super limited. I mean you can argue for a Smiter… but lets be real all that really does is save you like 2 minutes killing ubers. 99% farming ubers is collecting keys. The super cheap option of a Black Scourge does it just fine.
Fort can actually go on a lot of builds.
That said it does tend to eventually get replaced with enigma for solo farming, but still.