There is a bad way i used to do successfully; breaker bar along the 1st cam lobe to jamb against the block.
I think the safe method is: to jamb a long steel tool thru a flexplate hole to interfere with the rear block ledge.
Otherwise (never tried this one
a suitable rope fed thru the #1 sparkplug hole.
Got to wondering; if a visegrip locked onto the flexplate teeth ring would hold up to balancer bolt loosening, less so for tightening.