Soloed as a lvl49 mage:
Sheep Overseer Maltorius, Frost Nova the other two and blink to the Plans. Jumped down the valley afterwards.

Just be prepared i case Polymorph or Frost Nova gets resisted.

Soloed as engineer rogue... 1 sap... 1 Blind and 1 Net-o-matic... :) Take it and then jump!

A rogue with imp gouge can solo this... barely.

Sap the overseer, blind one of the sentries, then stand right next to the plans and gouge the other sentry. RIGHT after you gouge, loot the plans. You have to be extremely quick, as the gouge will last almost exactly the same amount of time as it takes for you to open the plans. Popping evasion before gouging might help - if the sentry breaks gouge before you finish, you won't get interrupted if you dodge (I think). Then just vanish and be on your way ;)

I was able to have my pet draw aggro, then I ran up, collected the plans, and jumped off the edge to the pit below. Pet died but not me!

The Slag Pit is the second cave from the left for those of you without co-ords. The plans are behind 3 elites on a balcony. They're not soloable unless you plan to grab the plans and make a quick escape.

The easiest way I found to get in was to go in the far right cave, run across the bridge and thru the big room and to the left.

These guys are impossible to kill, I tried as a 55 druid but failed miserably doing little damage.

Did it quite easily as 53 hunter, send your pet to its death but make sure you hit mend pet before he goes, that way he will be getting healed while he is dying and gives you more time to grab plans and get out.

At first I was going to write out some crazy idea about how a Shaman MIGHT be able to solo this, but it relies on luck and ridiculous timing. It's really not worth the trouble.

So, to put it simply, this is extremely difficult, if not impossible, for a Shaman in their low 50's to do without at least one partner. Stoneclaw Totem just doesn't last more than a second or two against these elites.

If you're a Shaman and really want to complete this quest, find someone else to do it with.

I have done this quest several times on various characters, where there are multiple mobs on that ledge. But just now, did it with only the overseer there. After getting the plans, I waited on the jump down ledge for 5 mins, with still only the overseer there. They may have toned this down so it can be done solo at the suggested level for the quest.

Soloed as lvl 54 protection paladin. I pulled them and ran away, stunning one of the adds. When the other two gave up chase, I quickly ran back and kept the stunned mob in combat. After I killed him, I did the same with the other add and then I was left with only Overseer to kill him 1v1.
Even though it was not easy to kill either of them solo, just keep up seal/judgement of light/wisdom to replenish health and mana. Each fight took about 2 minutes. If you are blood elf, use mana taps along with arcane torrent to replenish some extra mana. Even the sentries have mana, though they don't cast any spells. And arcane torrent is a nice extra interrupt for those 400ish shadow bolts.

When I was left with one elite to face, I actually managed to loot the scroll, just stun the mob and go for it. If the last mob is Overseer, you get extra 2-3 seconds as he probably starts casting a shadow bolt.

Sheep Maltorius, nova two adds, grab plans, Blink, leaving combat.

When I did it (Patch 2.3), There was one The boss and a normal lookout (lvls 50, 47) and normal, so I just killed them for the Wanted Quest then grabbed the item, piece of cake. I soloed it as a 48 Hunter.

A very easy quest after patch 2.3. Non of the mobs are elite. Did it as lvl 51 warrior - easy to take down 3 mobs at a time! Absolutely no problem.

as of 2.3 i think the elites are gone and Malfurious doesnt have guards anymroe correct me if im wrong

great information here execpt the bosses were no longer elites....im a lvl 49 warlock and took them out with ease with my voidwalker out...one is a lvl 48 and then the guy you have to kill for his head..the oversear is a lvl 50 adn the plans are right beind them on the table

Cleared this easily as a lvl 50 prot pally. The fastest way to get in is to jump down the hole and go through the gate, take a left and keep going. Just two guys, a lvl 47 lookout and the boss, no elites in the area.

To reach the cave entrance is a bit hard!

I made the mistake and I tried to reach from the bottom of the pit and was looking for elevator for a few minutes and I couldn't find one!

I then thought to jump from the surface down to the cave entrance at around 49.7, 45.4

It works!

The entrance to this cave is at 49.76, 45.76 for those people like me who just run to coordinates you'll find the above coordinates only help if you realize you must go into a cave.

The room looks empty on the screenshot?

Item required, [Secret Plans: Fiery Flux], is exactly at 40.4 ; 35.7