Flame Leviathan (10 man) has officially been 5 manned :O
1 Prot Warr, 1 Resto Shaman, 1 Holy Pally, 1 Ret Pally and 1 Blood DK. (not that it matters for this fight)
Strat: 2 Demos, with passengers and 1 Siege driver only.
Simply DPS him with AA guns and Pyrite and he goes down fairly quickly.
Feel free to check my armory for the 10 man achievements, unfortunately no SS available from the fight, did get one after he despawned though.
5 man FL: