How can I remove the wall fitting where the sink drain line goes into the wall? Because of the cramped/limited space under the sink I can not get produce enough torque to loosen the fitting.

Ohhhh boy. A problem we plumbers deal with frequently. What tools have you tried so far? Channel lock is the first option. Next comes a pipe wrench. If that's a no go, there are few options. One is removing the vanity/sink so you can get better leverage, or you may have to cut the nut or piping, or open the wall, etc, etc. Let us know more about your situation, pics may also help. Trust me, it can be done.

How can I remove the wall fitting where the sink drain line goes into the wall? What are we talking about here? A threaded cast iron tee with a brass Midland bushing or a PVC threaded adapter? Is the fitting still connected to the drain pipe? Back to you, Tom