I might have missed this but try taking the first cog or two off (you need two chainwhips to do this). Gives more "freewheel key" area for your wrench to grab. More secure. Hit said wrench with a hammer too. I don't know if the shock helps or what but that's what I did when I didn't have a vise.

and then note what nlerner said about taco'ing a wheel. Hammering on the wrench could dent the rim.

good luck
cdr