There is a Priest+4xWarlock team in my battelgroup that I have faced a few times in both arena and in the battlegrounds. His warlocks are all named after the Stooges, Lahry, Mohe, Curlhy, Shemph, IIRC. I'm not positive whether he's 5-boxing or has a partner (if I had to guess, he's 5-boxing).

The two times we've faced off in a BG situation were both EotS BGs, and my side handily owned theirs. I was able to pick apart his team at will if needed, although I generally just rotated through other nodes to make sure we held 3 towers at all times. The key for me against him in the BG setting was to avoid running in from max range slowly and giving him time to apply DoTs. Cleanse helped dramatically as well. Death Grip + Snares was really tough on him, once I trinketed a fear, I could just mutilate his poor clothies with my melee team.

In arena, I remember losing one of our early fights together, but I think I've won since I acquired just a little bit of gear. I have a pretty horrible arena record right now, so it's notable that this multiboxing comp is actually one where I have had at least a success or two. I'd much rather face it than a "normal" team, if that tells you anything...