Demitrik

After killing the Nayt the Gatekeeper, break down the wall behind him. You'll find a teleporter. After using it, you'll see a bridge, walk across it. On either side there is a group of 3 mobs, clear the area. To the North there is another bridge that goes down with Dimitrik and "A Toxic Contraption" and the end. Assign someone with a high runspeed the task of "wrench runner".

When the Contraption is sitting at the bottom of the bridge, Dimitrik will port down to him. Every tick (and it happens FAST) heals him for 4% health. In order to make the Contraption leave the bottom of the bridge, you have to throw a rusty wrench at it. Dimitrik ports himself after the Contraption's emote. As soon as the emote hits the wrench runner needs to simply spam click the Contraption. If done fast enough, you can stop all healing.

Have your tank walk to the Contraption while the rest of the group waits mid-bridge. Your tank should right click and "Disarm" the Contraption which will aggro Dimitrik. Your wrench runner should be waiting off the bridge at the top running between the east and west sides of the platform looking for a toolbox. After harvesting a rusty wrench your wrench runner needs to bolt down to the bottom of the bridge and wait to hit the Contraption. After s/he does, Dimitrik may aggro him/her, so the tank must be ready to taunt him off. Have the runner run back up and start looking for a toolbox.

Rise & Repeat.