First off, no need to get the bloody skulls to drop, just use a grinding weapon on the wither skeletons to get the right kind of bits. Take said bits and combine with another kind and an upgrade core IIRC to get a thingy you need to chuck on the ground and light on fire, then right click with your force rod. Gets you a bottled Wither. Best part of all? FREE FORCE FLASK AFTERWORDS!