either make or buy a locking tool. or you can use an impact wrench to get it off.
some people turn the engine over with a long bar in position to crack the bolt, but get it wrong and you can cause more problems or hurt yourself in the process.
i've made a locking tool which is basically a large bar that bolts to the front of the pulley. Be careful yanking on it as you don't really want to spin the engine backwards as its common to get the timing belt to jump teeth as its designed to tension spinning clockwise.