if you havent ot an airgun get a socket or big ring spanner on the bolt and get a big club hammer and whack the spanner so it shocks the bolt loose.
another way to do it is to put the spanner firmly over the nut, turn spanner clockwise so it jams on something like the lower arm for example, then disconnect the coil king lead and crank the engine over for a split second, this will also break the bolt loose (but make sure its the correct size spanner or socket, and that its firmly on the bolt)
didnt ask! i take it you only want to remove the pulley to change it! and that the engine still has timing chain and everything else fitted? if the timing chain or tensioner or cams have been removed then you will mess the timing up.