I completed an Insane/RL run with a Razorlock/tentacle build and it turned out to be quite good. Even though it's fairly similar to my Steamdemon build, I might write a separate guide due to the quirky nature of the mechanics. The character killed Elandar in 3 attacks and Argoniel in 3 attacks after that (using Link of Pain from The Black Plate). I tried Atamathon for fun, but it's been so long since I've attempted that fight that I totally screwed up and died in dishonor.
The crux of the build is Ogre-wielding Razorlock with the tentacle offhand, which fundamentally changes how Razorlock's multi-hit works. Normally, Razorlock does 4 hits per bump. If you Ogre-wield Razorlock with another steamsaw in the offhand, they only do 3 hits combined per bump, which makes that setup inferior to just using Razorlock by itself. However, the tentacle offhand interacts strangely with the "dual-wielding" component of Razorlock, causing it to do 6 Razorlock hits and 2 tentacle hits (8 total, double the normal amount for Razorlock). Tempest of Metal attacks with both hands, so you do 8 hits to all adjacent targets as well (including the primary target). 16+ hits per bump is pretty crazy. Projection weapons seem to have a weird interaction with tentacle hits, but in any case the projected attacks also do multiple hits because they use your mainhand and offhand weapons, regardless of which weapon caused the projection.
With Flexible Combat, Expose Weakness was adding over 300 weapon damage and well over 100% resist penetration (more like 400%, not that it matters). The build can do upwards of 26k damage per turn against the training dummy, although it seemed to be much lower in practice, perhaps due to armor. If anything can actually survive that damage for more than a few turns, the bleeding stacks up quite high as well. I was also using Corrosive cone, which didn't deal that much damage per proc since I didn't have much scaling for it (and it's affected by the Ogre-wielding offhand proc penalty), but the sheer number of procs and the AoE overlap made it quite good for clearing rooms. The leech from Suffuse Life was giving upwards of 1k healing per turn, so I was quite durable as well.
I'm trying to figure out a build that could leverage this to beat Madness difficulty, but the category points are really tight. The build in the posted link doesn't have a way to remove enemies' effects/sustains apart from dispersion gloves, so I think it would need tweaking to be Madness viable. It's very strong on Insane though.