Pure muscle is the only way.

The impact wrench isn't very effective when the object the fastener is attached to can move. In this case, the steering shaft isn't firmly fixed, and when the wrench "impacts", the shaft jumps a bit, taking most of the power out of the impact.

I had the same issue when I needed to replace the turn signal switch on my '92 300SE. It has some sort of socket-headed bolt (torx or hex, I don't remember). I tried and tried and tried and almost ruined the bolt.

Took it to Silvio's - the fellow that does all the wrenching I can't do. Ralf, the 6-6'' shop foreman got the right socket and a 2+ foot long breaker bar and muscled it right off.

I could have done it myself as it turns out. I was scared I was going to damage the steering lock so I never tried very hard. He said they're practically impossible to damage.

Having said this, if you snap your steering lock - don't come looking for me.

I'll bet all you need is a long wrench, good angle on the fastener and a fair amount of upper body strength. You can substitute longer wrench for lesser strength. Get the arm of the wrench as close to the rim of the wheel as you can - short extension and/or deep well socket to limit the length of the extension. Grab the wrench at the pivot point with one hand, and at the end of the wrench with the other and PULLLLLL! (Or Puuuushhhh, depending on where the handle is...)