You have a few options. Use an impact wrench while gripping the shock shaft with vice grips and a rag (so you don't damage it). Use an oxygen sensor socket so you can stick an allen wrench in there and use a socket wrench just like normal. Or use a regular socket (may need a thin walled version) and use a pair of vice grips instead of a socket wrench so that the hole is still open and stick an allen wrench down into the key on shock shaft.
O2 Sensor socket
MacGyver