Actually, disarming him of his staff would be particularly effective. As his bonded item, he requires it in his hands to cast a spell. Without it, each spell requires a fairly high concentration check.
See, that shows my ignorance in how casters work in pathfinder. My interest is usually in melee characters because I find them a bit easier to auto-pilot through combat to get back to the fun bits in between.
Regarding the readied attack option, yeah, that's what I meant with the chess game: at higher levels along free-hand fighter and the disarm feat tree you can toss their weapon a few squares away, they have to move to get it, and trade their attack for a move to pick up the weapon. Problem is, then their out of swing range, so you close, rinse, repeat... It seems to me that if you can chain a foe into the cycle they're spending their actions not hitting you with their full attack routine and your friends can put them down. But there are other concerns: as the French Giant, having specialized in fighting groups of people for so long, said: fighting groups of people is completely different from fighting individuals.
Enough about my character ideas though, I can't wait to hear the outcome of confronting the next roadblock in the adventure. Unfortunately I've still not finished episode 8, so I've my leisure cut out for me...