If a drain pipe won't loosen, and is still solid (not falling apart from rust), one fix is to cut the end off, removing any stuck nut or fitting. Then use a rubber coupling, or "fernco" to join the sink drain to the wall pipe. A hacksaw would be sufficient cut the pipe, but I'd use a sawzall cause I have one and they make quick work of it.

Rusted steel drains really don't want to come apart, even when standing and jumping on the pipe wrench. So my usual approach is to cut the pipe, and attach PVC with a fernco.