This is actually factory recommended for Volvoes (who only started having harmonic balancers about 15 years ago) and often used by SAAB mechanics, too:

First you disable the ignition or diesel fuel supply.

Then you get your socket on the nut with your longest breaker bar and cheater pipe wedged and bungie corded up against the chassis somewhere so that it can't turn in the direction of engine rotation and the socket can't pop off the bolt head.

Finally, you just hit the starter switch a few times and it'll usually spin the bolt loose.