Forget about Fortitude, you won’t get Lo for a while.
Plan on running Smoke runeword on yourself and possibly also your merc or a while, unless you have a unique or rare armor that is better.
The main thing is, you may well need to farm Nightmare for a while first. You can often get to Hell in your 50s or low 60s. But you can farm Nightmare Chaos Sanctuary and The World Keep into your high 60s or low 70s. This can be worth it if you are doing solo-self found, which you really should your first time through IMO.
Don’t worry about Sunder Charms. They won’t be worth it until you get much better gear and you can’t use them until level 75 anyway and as you say you have to defeat Hell before you can farm for Sunders anyhow.
You are better off sticking with Cold+Fire hybrid. I’m not sure how you speced it, but I leveled with Frozen Orb and Hydra and that was pretty effective and safe.
For your merc, an armor like Goldskin can be good for a while. Another good armor can be the set item Griswold’s Heart, which has three sockets you can put low level runes in to get better resistance (usually Lightning, Fire and Poison). Both should be upgraded. The runewords Treachery or Lionheart can also be good, though they are harder to make than Smoke. Even better are Shafstop or Guardian Angel, but only when paired with a helmet that provides additional resist all (since they provide no resist themselves).
Any armor for your merc that has base armor in the 400 range would be good at this level, then when you make the runeword it will imrpove. With +defense you shoudl be look at defense in the 1000+ range for the merc.
Also, since you are a Sorceress, you have to keep teleporting away to pull you merc out of trouble so he won’t die so much, and also keep feeding him potions, which you can do by using Shift+number, i.e. Shift+1 to give him a potion for you #1 belt position. You find lots of potions, so don’t be stingy.
Are you using Instant Cast? I highly recommend instant cast. My setup for Cold+Fire Sorc is something like:
Frozen Orb on Left click, Hydra on Right Click, Teleport mapped to Z, Static Field mapped to X.
Hold Shift + Left click to cast Frozen Orb at will, use Z to teleport around, drop hydras whenever. Rotate Orb>Hydra>Orb>Hydra, etc. teleport and run away. Teleport a lot to keep merc out of trouble.
For your gear, ideally you can find Tal Rasha’s set, but don’t count on getting all of the pieces before going to Hell. It won’t be worth using until you can get at least 3 pieces. The head piece is good by itself for your merc.
You can farm for Tal Rasha’s in NM Act 2 by running Canyon of the Magi and going to all the tombs and killing Duriel, but it may not be worth it, you may be better just running Diablo and Baal.
As a Sorc you don’t need “defense”, i.e. armor rating. You should probably just stick with Smoke until you can get Tal Rasha’s or something like Skin of the Vipermagi. Also look for Charms that give resistances, you’ll need those in Hell. Don’t worry about upgrading Skin of the Vipermagi, its not worth it for a Sorc.
For your merc he’ll need both defense and resistance. He’ll need armors with 30+ Resist all until you can get good head pieces with resist all. Ideally in Hell he’ll have either Treachery or armor with 1000+ defense. Good NM level armors, like Duriel’s Shell or Shafstop can be worth upgrading for your merc, but be careful to check the upgraded requirements and that he’ll meet them.
Once in Hell, I recommend running the Forgotten Tower over and over until level like 75-80 if you are struggling. Use Magic Find gear and see what you can get while getting runes from the Countess to upgrade. I got a nice Skin of the Vipermagi in the tower and got 2 pieces of Tal Rasha’s there by running it with MF gear while in my 60s-70s.
If you get Skin of the Vipermagi with at least 30 resist all, then I’d socket it and put an Um rune in it when you can find one. That will give you 45+ resist all. If less than 30, then I wouldn’t waste a socket on it.
If you get Venom Ward (pretty common), that can be worth upgrading as well for your merc to run Andriel over and over to farm her in NM and/or Hell. She’s an easy kill with Cold and Fire.
Don’t forget about Crafted items. Save those blues and make Caster gear. Keep all of the gems you find and when you farm the Countess you’ll get lots of low level runes and you’ll find blues along the way. Keep rolling Caster gear for any pieces that you don’t have good upgrades for. Don’t worry about the level of the base, since defense is meaningless for a Sorceress in PvM (don’t get hit).
Let’s talk shields. You can find White Monarch shields in Hell Act 1, but they aren’t as common as they will eb in Act 2+. Still get a Monarch ASAP and socket it to make Spirit. Until then, run Rhyme or Splendor. If you are in NM then you can focus more on Magic Find and may not need the resistances. So you can use options with faster cast and faster block. In Hell you’ll probably need resistances on your shield.
For weapon I assume you are using a Spirit Sword. If not then that’s an obvious upgrade. Spirit Sword will be your best option for a while, until you can get full Tal Rasha’s or Heart of the Oak. Technically its possible to find The Oculus in NM, but don’t count on it, lol!
As for Amulets and Rings. Keep every one you find. Either make Caster items from them or re-roll them in the Cube.
Oh, and to kill the Countess here are some tips. She is immune to Cold and Fire, so you can’t kill her directly. Hopefully you have Hydra. Fill her room with Hydras to kill all her minions. May need to throw Orbs in there too to clear trash. Then teleport in on top of her to have your merc attack her while you back off and cast Static Field. She’ll drop quickly if your merc has a half decent weapon.
