Recently I needed some small radiant shards for 2 fiery weapon on my stinging viper rogue twink :) so headed off to Scarlet Monestary, only had one level 60 on the server which was a priest, I already had enchanting. Previously I had soloed him with my pally on a different server pre talent rework. I died once and also didnt like how close it was, before I got the hang of it. I don't have very good gear and am shadow. Anyway enough of the babbling and on with what I learnt. If you know how to get to Herod skip the first 2 paragraphs.

Take the standard route to Herod, I blow most of my mana killing the first 2 guards to get them out of the way quick, your mana will have regened before you get to herod. Go through courtyard, down the stairs, watch out for the mage patrol (1 single elite, but they can walk off and get help if dont kill quick), I sometimes kill her, sometimes not, if you do kill her go on the stairs and you wont get adds. Keep going forward, past another mage patrol, then jump over the cannons to the left, another mage patrol to watch out for, then just keep going forward and onto the last segment, I think theres another mage patrol. In the last segment there will be about 3 elites on the left hand side, with a bit of pillar sticking out on the right side, hug the wall on the right side and you wont aggro. Then open the door, don't get too close to the elites near it though, go through and theres Herod.

To open Herods door you will need the scarlet key, you get this from a lockbox in "The Library" near Arcanist Doan, I soloed this myself but its a big pain (for a priest), you tend to get alot of elite adds and the scarlet monks put your spells on cooldown. Doan himself is easy as all his spells resist. Im pretty sure you can even get the key without fighting him, but he drops some nice stuff.

Now for Herod, he has too much health to just kill by using all your mana on him, I think even epic wearing priests would run out of mana. Have a look around the room, there are chairs right in front of Herod, I'll explain them later. First I use PW:S, then Mind Blast him from the top of the stairs, then start wanding him. Pretty soon he will be doing a whirlwind attack and he will be immune to all spells, including a SW:P on him, so I don't use SW:P at all now. When he does the whirlwind run away from him, as its just going to hurt you. Basically you just keep doing this, when he comes out of whirlwind you mind blast, shield and wand. By the time he is down to 50% you should have about 80% of your mana left or more, as this is where it gets harder as he goes into a rage. Now head to the top of the stairs, apply a new PW:S whilst he is in a whirlwind, we want the debuff to wear off as quick as possible. Continue with mind blast and wanding, he will break through your shield pretty quickly, if shield debuff is worn off quickly reapply another one, then you can mind blast him again and carry on. If you still have the debuff and you have no shield just walk off the side of the stairs to the floor, it will take him a few seconds to get back to you which by then you will be able to apply another PW:S. When he next goes into a whirldwind attack just go back to the top of the stairs and do the same. Like most bosses he is immune to fear.

I usually finish with about 30% mana, loot him quickly, press some macro's and restart. As soon as you have looted him, head towards those chairs I mentioned, when all the trainee's come in you won't aggro any of them. Reset instance and repeat. I only recently found out that standing by the chairs won't aggro the trainee's, I killed them all once but they are pretty hard for a shadow priest with no aoe, a holy priest might be ok here as they have an aoe I think. I generally found it was too much of a pain to kill them all, especially as I didn't have much mana left after the Herod fight. The macro's I used, one was from this site

/invite Player
/fbdoin 20 /script PromoteToPartyLeader("party1")
/fbdoin 20 /script LeaveParty()

Then

/target Yourself
/cast Power Word: Shield(Rank 10)
/logout

You need a mod called Flex bar for the first macro to work.
Anyway now I found out that you can stand somewhere you don't aggro the trainee's you can just do it at your leisure, invite your friend to group, make them leader, leave group, logout whilst its counting down and log back in again at start of instance.

Each run would take 7-8 minutes, after the 5th run I get ported back to IF where my heartstone is set, yet it doesnt put a cooldown on it which is pretty neat!
Priests can also easily solo Princess and Tinkerer in Maraudon, if anyone wants I can write how to do that too.