I first solo’d Scarlet Monastery at level 38. It’s not something I recommend, as it’s much easier and more efficient for players to level through questing, especially with the amount of XP you receive from quests in “new” Azeroth. It’s not easy, and you can really only solo the first sections of Library and Armory. Cathedral is too hard at this level. Graveyard might be doable, but I never ran it.
First off, at level 38, you need to be prot spec, of course. I believe THIS was my build. The key talents are Reckoning, Redoubt, Blessing of Sanctuary, and Improved Righteous Fury. Important to note is that the tool-tip for Righteous Fury is broken and does not update when you put points into Improved RF. This might freak you out because you feel like you just wasted 3 talent points, but it’s there.
It’s been a long time, so I can’t give specific gear info. At this point, I had level 70s running me through instances, so a lot of my gear was blue mail items from various instances. As long as you have decent stamina though, you should be okay. In addition, a quick 1H sword and shield is a must. Aim for a 2.00 speed sword. Also, you ABSOLUTELY need a shield spike. If you can afford it, throw a thorium shield spike. With redoubt up, you will be doing a lot of blocking, so you want to be doing additional damage from that as well.
Keep in mind that I play as a blood elf, which gives me access to Arcane Torrent. Arcane Torrent is INVALUABLE as a prot pally, because basically your only interrupt is Hammer of Justice on a 1 minute cooldown. There are a lot of healers in SM so it might be impossible to run this as alliance at this point.
At this point, we zone into the instance.
Make sure righteous fury is up as well as retribution aura. Buff yourself with Blessing of Sanctuary and activate your Seal of Light. A lot of people switch up Seal of Light and Seal of Wisdom, but when I was a low level pally, I mainly used Seal of Light to keep myself going.
The key is to fight packs of mobs—but know your limits. Two is ideal, three is a dangerous. Especially with all the Scarlet Protectors healing, it can get pretty hairy. Run up to the first group of mobs and judge Seal of Light then reactivate it. You should pull the two mobs. Line of sight is very important, so I usually run to the entrance. You also want to position yourself so that when the Scarlet NPCs get low on health they don’t pick up adds when they run.
Go slowly at first. Your damage isn’t great. In fact it’s a pain in the ass. Get ready for some LONG fights. The key to these fights is doing whatever you can to minimize the NPC’s heals. This means using mana tap to drain, and HofJ and Arcane Torrent to interrupt.
You should be able to clear out the first area easily. I don’t remember if I took out the Houndmaster Loksey at this level, but it’s not impossible.
The brick wall I ran into at 38 was casters. Casters are the bane of a protection paladin’s existence! When you start fighting the Scarlet Adepts and Gallants or whatever those bastards are called, you don’t have enough DPS to take them down, due to the frequency of their heals.
Overall, soloing SM is neither efficient nor profitable until you reach your mid to late 40s. But you can solo it, the first parts at least, as early as 38. SM is actually my “fallback” instance whenever I’m running low on gold, I need enchanting reagents, or my weapon skill needs work.