When you activate the power in "FIVE" quickly go back to the starting room and get a "Quick Revive".

Then kill the last of the Zombies. In the 1st or 2nd round after you activate the power, the thief should come and get a good view of the stairs (1 floor down from where the starting room is).

Get a grenade and down yourself, then after that you should have two Pack-a-Punched pistols and you should kill the thief easily, thenalso be revived!