Are you sure you used a strong enough impact gun?

I haven't tried these methods myself, but I can think of some ways to get the bolt moving. One is to jam something in the flywheel to keep it from moving while trying to turn the dampner bolt (easiest when the engine is on an engine stand). The other is to remove a spark plug and jam rope to take up the volume. This would use the piston to keep the crankshaft from turning. Another method people use is to mount the socket and breaker bar under the hood in such a way that you can bump the starter and use the engine to start the bolt out.

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95 E320 Cabriolet, 169K