What about Ethereal Jaunt or Etherealness ?
Renders him invisible and able to go through walls, but he will be still near the action if he chooses to.
So that he can choose freely to stay and bring the assassin down or to flee and gather more servants for the fight.
He could also use some buff spells while ethereal I think.
Max on Contingency is level 6 - Jaunt is level 7 and pure Etherealness is level 9