First try using the spindle bolt or screw of the same thread to lock the tool to the cup so you can torque the haill out of it without is popping loose. Impact wrenches work wonders with this. If all is OK, you'll either get the cup out or totally shave off the splines.

When all else fails, if the rings are aluminum use a 1-1/4 to core out the face of the lockring, and drive the cartridge out from the other side. Once that' done, it's grunt file work to remove the remainder of the ring, and possibly (most likely) clean up the threads with a BB tap. IMO, this is only warranted if the frame and overall bike quality is high enough, and from the sound of it this won't be.