I stuff a clean rag into combustion chamber via sparkplug hole to the point when piston compresses rag before TDC and then can safely wrench it off.
A large strap wrench can also be used around flywheel.
Briggs makes a couple of sizes of flywheel holders but may only work on their engines because of the way it grabs the fins. http://www.cometkartsales.com/store/tools/images/newphotos/flywheelholder.jpg
i just used a breaker bar and held the flywheel and gave the wrench a smack and it came loose. just like when you need to take the bolt off the snout of a crankshaft on a car