Marauder Unlock @ 14

[Omnivore]

Yep worked for me at 11, 2nd floor of maze in one of those "fell auras" (+40% crit dmg, -20% all resists). Now I can quit playing a human flash bang grenade! Although it is kinda hilarious playing a sneaky lightbulb.

[duskhorizon]

For some reason I couldn't get 2 poisons from my weapon when using 2hander so it worked for me on ~18 level.

Died few minutes later so well that was about time

[vonfackenheim]

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Pulled it off at 12 using the lightblade build. Crimson crystals must -100% resistant to light.

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[ArschAngel]

I actually unlocked him at level 9 with the lightblade build.
my skills: http://i.imgur.com/82gLYQk.jpg
I was saving generics for Staff Combat and I didn't even need it. I had Kor's Fall which I channeled to light damage (of course), I had Summertide Phial (which is another +10% light IIRC) and some wizard hat that gave me another +10% light damage. In Kor'Pul I encountered some treasure chest boss and this is how it went:

[The Tapir]

Tried both methods detailed in this guide, and unlocked the class after much stress in not finding a red crystal until the last quarter of the map on the 3rd level of the Scintillating Caves. Don't have a picture of the unlock message, but kudos to those who spent the time to figure out these methods - they both had clear paths of progression toward the unlock in gameplay.