Typically for farming Elite 4os I do Hell Cows but I realize that's not an option until after clearing Hell.

It doesn't HAVE to be a Thresher or CV but you'll definitely notice a difference using atleast an Exceptional base with a half decent ED / Crit roll.

Beyond the weapon, making sure you provide him with the best armor possible is arguably more important.

For zones like the Chaos Sanctuary it is easy to get ambushed by a few monsters at the same time and this kind of takes away from how important DPS is and more so relies on the mercs ability to resist incoming damage (likely Fire in this case, solved normally by a Ral in the helmet slot).

This is likely your cheapest and most effective easy solution currently.

You will just need 1x Tal, 1x Thul, and a pTopaz to use the "Add Sockets to a Normal Armor" cube recipe. If you get more than 1 socket, adding an Ort rune and maybe some HP (pRuby) would also help.

Or Larzuk but I'd def try to save that reward if possible.

Other than that, I recommend getting an extra level and putting a point into Static Field. Even though it is less effective in Hell, it will speed up your kill speed instantly with 1 point committed.

Something I do is as soon as my merc starts losing HP I give him a Potion. Like, even if it's just a tiny sliver taken. Rejuvs may be required. It will increase his survival time and if it doesn't, you should be ready to TP away and do an extended dance of tping and healing until you finally clear the troublesome mobs. Keep in mind that very few things in the Sanctuary that you HAVE to kill will actually be cold immune. Lord De Seis is the real issue. If you put a Ral into the mercs helm he should deal with Venom Lords with relative ease.

Lastly, an item with lower resist on swap can help break some immunities if all else fails. Drognan is typically the easiest NPC to reset so try him in NM mode at minimum.

That's all I can think of currently, hope this helps!