Over the course of 50 rounds, the treant never gave up on chasing the mobile PCs to spike the paralyzed ones before they woke up? Somehow, I don't think it'd happen that way in our game here
The 2 mobile PCs were a wizard and a rogue. The wizard got a
ray of exhaustion through the treant's SR (the only spell he got through). This reduced the treant to half movement. The wizard also
fireballed the grove of trees in the cavern to destroy them before the treant could animate them.
[This was a Large treant, not Huge like in the MM -- it was stunted, living in an underground grove.]
Since the group had specially prepared and equipped for this battle, they (especially the rogue) were stocked up on holy water (half-fiend treant is an outsider). While the gnome wizard on dogback tried to get some spells through the treant's SR, the elf rogue nickel and dimed the treant with vials of holy water. The treant tried catching them, but through good tactics, faster movement than the treant, and a large cavern to manuever in, the 2 PCs stayed out of his grasp. When the treant went after one, the other would drag a friend out of the cavern.
The treant tried using objects around the cavern as missile weapons, but he couldn't hit or hit hard enough to take either down. After several rounds it became apparent to the treant that he wasn't going to be able to catch either of his enemies. He would have used a downed PC as a hostage, but the mobile PCs had wisely made getting the paralyzed folks dragged out of the cavern their first priority. The treant couldn't reach anyone down the smaller passage.
The win was a result of good tactics, some luck, and the perfect combination of 2 PCs surviving the initial
blasphemy (by being too far away). If the wizard had not gotten the
ray of exhaustion through, the rogue could not have run circles around the treant. And if any other character but the rogue had been the partner to the wizard, their slower movement rate would not have kept them out of the treant's reach.
Now, for those armchair/Monday-morning quarterbacks here, I've not detailed the entire battle. I'm sure someone could point out a flaw -- there's bound to be 1 or 2 in 50+ rounds of combat. Plus, this took place nearly a year ago, so I don't remember every detail. But, although I'm not an intentionally killer DM, I don't pull any punches -- dice rolls and monster tactics are straight up and honest -- my Players know this, and they won this fight honestly.
Quasqueton