How do I remove a Mercedes A-Class crankshaft pulley?

Lock the engine by removing the starter motor and using a flat bladed screwdriver placed in the teeth of the ring gear. If you have an impact driver and relevant hex bit give that a try while someone holds the screwdriver in place. This may not budge the bolt so in my experience the best way is:

Using a cold chisel hammer the bolt to turn it anti clockwise, you will need several good hits with a large hammer, this method will damage the bolt but it is important that it is replaced anyway as it can only be torqued up once.