Am I correct that the drain line is copper and then converts to plastic with a threaded adapter just ahead of the trap? That's what it looks like.
In this case, most likely the drain fitting is glued to the pipe. The drain fitting may have a large nut on the bottom of that remnant of the fibrglas base, but removing that will not help. You will have to remove all the ABS (plastic) all the way back to the transition to copper, and rebuild from there. It will not be difficult.
If I am wrong about the pipe, post back and we will go from there.