I see, thanks :) Can't post his char here atm since I don't have access to it right now but will try to solve that tomorrow.
What about this:
He trips them as they come within his reach on their turn thanks to his 10 ft reach. Then when it's once again their round they stand up again with their move action so they only have a standard action left which leaves them with the choice of either get into a defensive position (Total Defense) or use the action to make another movement. If they make a move and tries to get in melee contact with him again he gets another AoO thanks to his Combat Reflex in which he can trip them once again so that they still never make it past his 10 ft reach.
I know that one solution would be to attack with many weak opponents but it doesn't make much sense since his AC goes between 25 - 30 (and the weak opponents would be around level 4-5 since he's only a level 8).