Working in NYC, I've done a number of these...
I used to call in a plumber, but he eventually started answering me with "I'm busy, you can do this yourself, I showed you how..." which was a bit of a joke, since he's one of the strongest men I've ever met, and I weigh all of 140.
Where I'm going with this, is that I've never seen a fitting get broken, but I've often had a pretty hard time applying enough force to loosen the nut. It takes a LOT of force to get things moving.
Get yourself a long (5-6ft) pieces of black pipe to fit over your wrench handle - you'll need the extra leverage.
The pen is pointing to the part you need to turn - in this case, I'd need to crank up.
How are you planning to deal with getting the rad back in place, when your floor is higher?