In this video I multi-box a bear run. I used 1 Blood Deathknight and 4x Elemental Shamans.

Edit: This isn't a multiboxing first but the video is.

I had an item level of around LFR level (391) for my PvE Tank and Ele sets.

I used food-buff, hit elixir and a magic resist elixir on the Deathknight. The shamans only used class and profession buffs.

Server: Magtheridon - Horde (US)
Guild: The Zerg

Computer Specs In Video (near the end)