Quoted from Baiter:I tried heat, I soaked it with WD-40 three times overnight but that solvent doesn't seem to work on loctite (if that's there... none visible). I destroyed a flat wrench trying to use it as leverage in the magnet notch, tried the biggest flathead screwdriver you can purchase at the hardware store, but that slot wasn't made for a screwdriver. I can't find a spare coil stop, so I gave up and slapped it back on the game. At this point I'd rather waste money on a new assembly than wasting time and money at a hardware store trying to figure out how to work around Stern's lazy mechanical design that requires loctite.
For me, it was a combination of heat, large adjustable wrench on the nut, and a large vice grips on the core itself. Oh and also 2 and a half hours of swearing and giving up. (Had to pull it up through the top of the playfield due to mushrooming).