I used my lug wrent from the trunk, took the cap off the front of the transmission access hole, put the end of the wrench in there and braced it against the crossmember and the crossbar from the side body to the sub frame, this was on an SL, so I don't know if you will have the same points to secure in another car, there is also a tool you can buy where you remove the starter and bolt it in the place of the starter and it locks the flywheel.
But it costs $250 and usually has to be special ordered. I have seen some people make a tool that you can hold the balancer at the same time you remove the bolt. Good luck!
Brian