Whenever I had to take off the fan clutch with my old M103, I used an allen plus a deep 10mm socket on a 18" breaker bar. That (plus the special tool to hold the pulley) always did the trick - it's pretty hard otherwise with the clearance to the radiator and the torque on the bolt.

The clutch should be pretty stiff when you first start the car, but it should loosen up after a few minutes of driving (as long as the weather is cool).