First, the skills and points:
Conjure Nightmare
Conjure Phantasm
Empathy
Backfire
Accumulated Pain
Guilt
Illusion of Haste
Distortion
12 Illusion
12 Domination
If you are a Mesmer feel free to use whatever runes you feel like using to make it easier, but 12 in each attribute is plenty.
Next, the location for those of you who don't know:

*IMPORTANT* Unfortunately, this build won't work without a blessing from the Priest, so remember to do that.
After getting your blessing, follow the map above until you run into the group of Wardens clogging up a hallway. Run through them with Distortion until you get to their traps. Make sure all of the traps are deactivated then use Illusion of Haste to make your escape. Stonereap will be on the first right.
This part will require you to hide behind a wall, which sometimes will require you to pull the boss into range. It is important to note that the boss is only pullable when he is in the middle of the room, not behind any pillars, like so:

Hide behind the wall to the left of the door of his room and while he casts his degen spells (which are conviniently cancelled out by the blessing) spam Empathy and Conjure Nightmare on him. Cast Guilt when one of his hexes is about to end when you need energy as he will try to recast the hex immediately after it ends. He will go down pretty quickly if you keep Empathy on him at all times and cast Conjure Nightmare when you have the energy to.
After killing him walk through his room and right infront of you will be Stonewrath. This guy is simple. Depending on where he spawns you can hide behind this wall to avoid his only attack: Stone Daggers:

If he spawns in the water hide behind the opposite side of the wall or behind the last pillar of Stonereap's room, whichever you prefer.
Basically all you have to do for this guy is wait for him to Gale you then cast Backfire and Conjure Nightmare on him (you can add in Conjure Phantasm if you feel like it) until he has very little health left. Then use Accumulated Pain, making sure there are two hexes on him, for the kill since he seems to be just smart enough not to Backfire himself to death.
You might have noticed that Conjure Phantasm serves absolutely no purpose in this build other than just to stack on if you feel like it. If you find another skill works better than Conjure Phantasm does, let me know, I don't have the full arsonal of Mesmer skills on my Elementalist so there could be something better.
Happy Hunting.