Hi,
You have to make sure the balancer will not move at all or give. Put a wrench on your holding tool and run it up against something strong for support that won't give(frame or control arm). Then use a breaker bar on the crank bolt and put a pipe on the end of that. Turn the crank bolt counter clockwise and be careful because when it breaks loose it will send you flying! Its best on a lift so you can get a good angle and swing with the breaker bar. Sorry no real trick here, If you have the holding tool its just muscle...This must be the first time this has been off, I don't know why they get sooo tight. So the main thing is to keep the harmonic steady and not giving. Be careful!
PS, A strong air impact does not even work on these so don't bother trying that. A 3/4" impact will, but most shops do not have those as they are very expensive.