Stone should join in the attack after you take out most or all of the boss type creatures he sends after you. Do you have any attacks that do not need a target? (Firewall, or something) If you can do damage that might trigger a reaction, though the bubble is probably protecting him.
Did you clear all the other opponents, get the loot bags and open the chest by one of the cage doors (at the top of the other ramps)? There isn't much else you could try to trigger an attack by Stone.
Do you have a save before entering the arena, that you could reload and go through it again?
If not, or you got great loot you don't want to give up, try emailing support@larian.com, with a description of the problem and maybe a link to this topic (DKS
support page).
In XP the saves are in the folder
C:\Documents and Settings\ %username% \Local Settings\Application Data\Divinity 2\Savegames
or in Vista / Win 7
C:\Users\ %username% \AppData\Local\Divinity 2\Savegames
You may need to set Windows Explorer to show hidden files and folders (
XP, Vista,
Win 7).
