Name: Degen Oni Soloing
Type: PvE Solo Build
Category: Solo
Attributes:
Fast Casting: 10 + 1
Illusion: 12 + 3 + 1
Inspiration: 8 + 1
Others: 0
Skills Set: Ineptitude {Elite}, Kitah's Burden (aka Ethereal Burden), Spirit of Failure, Conjure Nightmare, Images of Remorse, Conjure Phantasm, Ether Feast, Distortion
Summary: This build has one purpose - killing oni in Mount Qinkai Explorable Area. I start at Breaker Hollow outpost, go outside into Mount Qinkai, then to the right up to the first oni spawn area. At this point one of four things can happen. If I'm careful and edge up to the spawn area, usually one oni will spawn. Then I can simply pull him back, cast kitah's burden on him, then conjure nightmare, ineptitude, and spirit of failure. This will give energy regen at +3 per miss from spirit of failure. I then use conjure phantasm and images of remorse for more degen/damage. Once the blind runs out, I use distortion until ineptitude is recharged. A second ineptitude and more images of remorse and conjure phantasm should finish the oni off.
If unlucky or too fast coming up to the spawn, more than one oni will spawn. There can be any number from 2-4. In this case I pick out an oni near the front and cast conjure nightmare on it. Then I run like heck back towards the rez shrine (which is broken, but it has NPCs around it and a statue of balthazar so I think it's a rez shrine). I stop running once the oni return back to their spawn point. I usually will cast kitah's burden on my targeted oni as it runs back, but this isn't necessary, it's just nice to get that 23 energy. Once they've run back, I follow and cast conjure nightmare or phantasm on the same oni again, then run back again. I keep on doing this, throwing in images of remorse when I'm low on energy and using distortion if I over aggro and they start closing to melee range. After a while (it'll depend on timing and the amount of oni there complicating matters) you should be able to degen one to death. If there are still three left, just do the same. If there are two, you can regen then use the same process except add in ineptitude and let them close to melee using distortion to escape most of the damage. If at any time you take too much damage, use ether feast to get some health back. If there's just one, then see above.
Once you can handle the first spawn with ease, it's time to move on to the next spawns. The next spawns are on a path to the left of the beach. In order to reach them you have to pass through the naga. You can avoid aggroing them if you're careful, but I got tired of this and find that rushing through them with distortion on can be fun. I rush through and drag them with me up to the oni spawn where the naga leave. Then the oni chase me until they lose aggro and I'm in a nice safe spot. Then I use the same technique to kill them as the first group. Then I continue on the same path to the next group and proceed to own in it in the same manner.
After this the run is basically over. There is another spawn, but it's risky. In order to get to it I cross the bridge next to the Aspenwood Gate and follow the path on that side of the gorge. I kill the 2-3 yeti (the warriors can take a while since they heal, but it's not hard compared to oni) and avoid the large group of yeti. I then curl around the hill and down to the oni spawn where I usually die because of rot wallowers which get in the way when I try to pull oni. It's possible to kill all of this group, but you have to be very aware of where the rot wallowers are in order not to aggro them and die a horrible fetid ground death.
Notes & Concerns: The main weakness to this build is due to drop restricts it's not really profitable. The first 1-5 onis won't drop anything depending on how bad of restricts you have. This means out of a total of 9-12 onis (and as many as 16 if you're extremely lucky) you won't be getting that many drops. In order to reduce drop restricts you can always start at Fort Aspenwood, kill the yeti, and do the other three groups in reverse. This works alright, just be careful of the rot wallowers.
If you really get into trouble with the first group or the yetis, you can pull back to the rez shrine where the NPCs will help heal you. The yetis usually follow you there while only occasionally will the oni aggro all the way to there. This really makes killing the yetis alot easier, especially if there are two warriors as they can dish out a lot of damage and you can't blind them both.
Drops: So far drops have been fairly mediocre. The best drop has been rare oni daggers, max lightning damage, +19% < 50. Other than that I've gotten non max white weapons, purple daggers, oni shrouds, regurgitated masses, lots of bones, montrous eyes, monstrous claws.
XP: 232 XP per oni. That's over 2000 xp per run, which isn't bad at all for a mesmer. I haven't timed my runs, but they don't feel too long. I guess 15-20 minutes.
Weapons: I use Gorrel's Staff. The extra 60 health doesn't hurt anything.
The build isn't perfect but it works. I ran several different builds in coming up with this one. I've tried soloing oni with a 55 monk and that doesn't work at all. So I brought my mesmer up and messed around until I found this. Running with accumulated pain or phantom pain instead of conjure phantasm can work as well. I expect a degen necro could run a similar build, but I don't have a necro and mesmers rock.

Credit: This build isn't based on any other builds - I tried the signet of midnight + epidemic build first on the oni (it's somewhere on these boards, I can't remember where) but it didn't work so I kept modifying until it became something completely different. The only thing left from that build is spirit of failure.
Feel free to comment. If you find a more efficient build I'd love to know about it. Or a build that can take multiple oni at once or take down naga as well as oni to prevent drop restricts, I'd really be interested in that.

I have some screenshots, but didn't consider them worth putting up. If you're interested in them, I can post them.
Slewfoot
New Republic Guild Leader
IGN: Slew Foot