Step 1: Clear to him
Step 2: A bear and pally works best vs him
Step 3: Buy trash can punch
Step 4: Have team sit outside of his room and have one person go in, bear or pally pref.
Step 5: The person in the room should use their proximatey to back him up into a barrell. This takes some practice, but after a while you get the hang of it. He doesn't like to stay "close" to you, so he will back up if you get in front of him. Just direct him backwards onto the barrel.
Step 6: Blow the barrel. AoE's work if your a pally, run to barrel and beckon works great with bear.
Step 7: Team charge in hit him a few times, team charge out!
Step 8: Rince and repeat.

*Side note*

The only reason he is hard to solo is simply because your tank elixer or trash can punch doesn't last long enough to kill him. So if you have someone else on the sideline, getting a tanking elixer every 4 mins or so, just in case you die, it will help out a ton for them to run in and either tank him till you get back, or they can rez you, run him out of room (or back him up to a corner) and then you revive in a place where you don't get 1 shot from his aoe.

Good luck!