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Doesn't the scenario suggest luring the Bane into a room where physical attacks can harm it?
The adventure does mention that if the encounter is becoming too static, you can have one of the Pechs enchant a stone, which temporarily weakens the ward allowing the PCs to be attacked, and grants them a stone to throw causing 1d8 damage..
Use the Secret Enemy chapter in the Playtest packet, a Drow woman threatens the Pech not to help the Deep Gnomes. She sneaks around and when she notices the Pech's trying to stop Bane, she kills one every two rounds. Letting the shield drop.
That last suggestion is pretty good, I might do that myself.
I don't think I'd change the encounter too much, except to use DM fiat to occasionally drop the barrier to attack, if things were getting a bit stale. What I would do is try to encourage the players to invest more heavily in magic before the encounter, forshadowing the magical immunity of Ogremoch's Bane.Btw that's an awesome name for a monster, I can't get enough of it.
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