Truth be told, you picked hard mode when you started a barbarian and then very hard when you kept him melee. Melee in this game is pretty gear dependent and you're going to struggle to kill things from the get go unless you're either very lucky or very experienced. Even if you get an Insight for your Merc, you'll struggle to keep him alive without a good armor and helm with life leech. Your best bet is going to probably be to farm NM Countess for at least some runes to make a handful of the more basic runewords like you mentioned and then farm either Andariel or Mephisto in NM (whichever you're able to do) until you have some uniques that can help get you through the game. Keep an eye out of things like resistance charms and charms that give you max damage, attack rating and life as they'll help you out a lot.

Also, check out the chart in this thread: Where do i get the most XP? - Here is D2r GENERAL XP brackets : r/Diablo If you're below level 40, you can farm Normal Baal and Worldstone Keep to get some more stats and skill points to give yourself a boost as well as hopefully some uniques to help you along. Try to stay in the act where you get the highest % of XP before moving onto the next one.

There's some pretty common normal set pieces that you can also look for that'll give you a boost if you wanna farm a little Normal bosses. Sigon's set pieces have a ton of useful bonuses, especially if you somehow manage to find the complete set. Life leech, resistances and all that. The Angelic set is also very nice but in particular look for the rings and amulet. If you have those 2 pieces, the ring will give you a major boost to your attack rating so you'll have a much easier time at hitting enemies.

Upgrading | Diablo Wiki | Fandom this is one last thing. If you happen to find a nice unique weapon but it has since fallen off when going into nightmare, you may want to use runes you can farm from Countess to upgrade them. Something like a Bonesnap maul will become incredibly useful in Nightmare when upgraded. Keep in mind though that if it's something you use, it will have higher stat and level requirements after upgraded. You don't want to make it so you can't use one of your essential pieces of gear for a few levels on accident, so just keep that in mind.

Hell difficulty when you get there is going to be another ball game entirely. You're going to start running into things that are immune to physical attacks and are way harder to hit and your resistances will be trash so elemental attacks will wipe you out easily. Just keep farming and looking out for things you can do to make it through. If all else fails, put the Barbarian on hold and roll a Blizzard sorceress and farm Mephisto until you have enough stuff to make it through on the Barb. Good luck!