The method I have found most effective is to take the fill nipple and gauge (normally) off the reg, clamp the reg in a vice between two pieces of wood to save it from damage and then unscrew the bottle, anti-clockwise, using a large 'Boa Constrictor' rubber strap-wrench. As above hot/boiling water may help release the loctite (though 242 shouldn't need too much heat) especially on the nipple and gauge!
Then clean up all the left over loctite, make sure the seal on the end of the reg is in good condition, reassemble, clean up the nipple and gauge threads and use some ptfe tape. taking care that none will get in the air ways, to seal them on reassembly. That ought to take care of it!