To build a list around Treebeard I would personally go all-hero. The problem with having one very expensive hero and a small number of troops is that a clever player will just concentrate on the troops and break you easily.
A nice themed hero list:
Treebeard
Aragorn "Strider" with Horse, armour and Elven Cloak
Legolas with Horse and elven cloak
Legolas is very low defence, but I figure he's better off with an elven cloak than armour as you'll be sniping with him.
Just for theme I'd be tempted to swap out the elven cloaks for Merry and Pippin in a fun game. But you're then getting back in to the "soft break target" territory.
To be honest, I wouldn't call this a competitive list, but I think it's close to impossible to make a bang up competitive list with Treebeard at 500pts. He's great, but he works better in big games. That's not to say it can't win- I think it'll pack a punch, but against some cheesy optimised list it probably won't stand up that well.
But it's not all about cheesy optimal lists I hope, is it?
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