It took a hell of a lot of force on my car. I had to torch the nut on the passenger side for a while before it would come off. I also ground down a socket to use a big adjustable wrench:

I used the round end of a crescent wrench on the allen as a breaker bar. I think even after all of that, and the torching, I had to remove the strut from the car, then lay the wrench on the nut on the ground and stomp on the allen to break it free.
Used LOTS of anti-seize to re-assemble.

Brendan