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I thought to post this one since it's so far the hardest boss I've encountered so far (38 now)
Most important thing is to stock up on 4K pots and keep the healer free from the massive amount of imps that spwan. One DD should focus on the dragon while the healer keeps the dragon buffed for healing. When the pitfiend spawns, kill it ASAP as well as that mob that heals him.
AsRock 990FX Extreme3
AMD Phenom II 1090T ~3.2Ghz
GEiL 16Gb DDR3 1600Mhz
ASUS GTX970 3x HD monitor 1920x1080