I'm not really resurrecting an old thread, so much as adding additional info to the subject.
The party has now fought two more of these treants (though without the 4 druid levels) in seperate battles.
To recap the old battles:
1st - 21 rounds - 4 of 5 PCs die, and treant survives.
2nd - 25 rounds - 1 of 5 PCs die, and treant killed.
Newest battles: (one additional PC these times -- 4th-level fighter)
3rd - 9 rounds - 0 of 6 PCs die, and the treant killed.
4th - 6 rounds - 0 of 6 PCs die, and the treant killed.
This group has learned from their original mistake, and has built on that lesson with some victories. It is interesting to see how they now handily overcome these treants. Each treant has been slightly different, and the party has gotten a surprise or two in each battle, but they've really figured out good tactics.
As most of the group are Chaotics, they haven't really sat down and worked out organized battle plans, but they each know what they are capable of, who does what best, who needs coverage for a weakness, and what the strengths and weaknesses of the treants are.
This is all a perfect example of a group becoming efficient slayers of a specific opponent. Its really cool to see this happen from the DM's perspective. I don't think I've ever really seen this happen.
Quasqueton