Note: I have included a map that is pretty self-explanatory. I'd look at that first before reading all this.
In Riverside Province, your mission, should you choose to accept it, is to get into the White Mantel's Fortress, retrieve the Scepter of Orr, then get out.
I didn't intend to solo this mission actually. But everyone else in my party died and I could not get to their bodies. Then we had a few people leave, and the rest followed shortly thereafter. Instead of going back to town, I decided to do a bit of exploring so I could find the best path to take for my next run through with a group.
All you really need for this mission is 1 skill that makes you move +25% faster or more. Also, at the very end, at the red dot on the included map, there is a level 20 boss mob you will need to be able to kill.
I used sprint as my quick move skill. It lasts a long time and has a relatively low cooldown. Also, being a warrior/monk helps because they are extremely difficult to kill. It should be possible with any class though. Well as long as they have a fast move skill. (I know mesmer would work with his "Illusion of Haste" skill. Still, it would be harder to pull off because mesmers are so physically weak.)
It takes timing and patience, but if you use sprint right, you can run away from all the mobs. They will give up the chase pretty quickly if you are sprinting away from them.
Soloing a level 20 boss mob is not easy, but it was possible for my level 17 warrior/monk. I have the 71AL + 20AL physical collector's armor set. I used Pure Strike, Bonetti's Defense, Galrath's Slash, and Sever Artery, in addition to my monk healing skills.
Note: Healing over Time spells might not work. One of the possible incarnations of this level 20 boss mob is a necromancer. He will dispel Healing over Time spells. When I finally beat it though, I fought against a melee knight guy, so bonetti's and sever artery and healing visage was ideal.
I should also point out that when I finally completed this, I was in a group of 2. I do not know if that affected the final boss mob. I think it did. When I soloed it, I fought against the necromancer. When I was in that group of 2, the final boss was the melee knight guy. From a warrior's perspective, this melee knight guy was easy to kill, but I couldn't kill the necromancer. (I really did solo this quest though, as my other party member was either dead or waiting for me at the entrance the whole time.)
So to get that party of two, just get a henchie, get the henchie killed, then go on to finish the quest as I have suggested. Hopefully that'll let you fight the easier melee knight boss mob.
Or, if you wanted to make people love you, you could get a group of 5 together, then just solo the whole mission for them. They'd probably be pretty grateful, as it is quick and easy to solo this mission once you know how to do it. (Note: this mission is rather difficult to do as a large group. It's just not possible to be as sneaky in a large group. Plus, there are a ton of guys everywhere. You have to pull rather delicately to clear them all out.)
Another idea had was to take out a group of 6 Warrior players. Make sure they all have sprint.
Then have one warrior solo into the White Mantel Fortress and steal the scepter. This will cause a cutscene to happen and the whole party will be teleported to the "get scepter" portion of the map (that point's marked on the included map.) Then just take all 6 warriors, and make a mad dash for the exit. You should be able to get through to the last guy with at least five of the warriors if you all have sprint and use it constantly. Then hit the last guy with everything you've got, kill him, and dash across the bridge to safety. I think this is sorta how you are "supposed" to do it.
Still, it's really just as easy to do it alone in the way I laid out. Maybe even easier.
I recommend following the exact path I laid out. Other paths are possible, but I believe mine is the safest possible path. Watch out for archer's with Pin Down. That will get you killed no matter how good you are.
If you try this and succeed, I'd love to hear about it. Suggestions and comments would be cool too.
R

Oh, and I had taken sprint off after Aurora for this mission.. wish I'd left it on lol.