We had one a few months back that lasted 50 something rounds, mostly the wizard (me) and the rogue against an Advance Half Fiend Treant, while the cleric, fighter, monk, and druid lounged around, being paralized by a blasphemy. I think the fighter was the first to come around, ...on round 40. I managed to get him to chase me by killing his children (immature treants) for a while, then he chased the rogue for a while when she began peppering it with holy water (we used something like 30 viles during the combat)
Don't know how we managed to all survive that one. Except for my riding dog. A CR 12 vs a CR 1 isn't pretty. I miss you Jake!!!!!
Miqaxd