I did it solo with 8 for the achievement very easily, and if you have 5 toons with GREAT PvE gear, ICC or whatnot, then it's probably doable with maybe 1 tank 1 healer and 3 DPS, but the problem is, he has an enrage timer of 10 minutes. I'm not sure you would get him down before that with only 3 DPS. i had 6 DPS and they were all pretty well geared in 251 PvE gear at the time, and it was a great composition, warlocks, shaman, boomkin.. al pulling decent DPS on him.

so yes, probably possible, but only if you overgear the content horribly.
Might be easier to just go get another boxer, or someone else to do some of the easier jobs like tanking or healing and bring DPS toons?