For that glow plug you can use a metal file or a hacksaw to cut the torn grooves away and leave two long flats so that you can use a normal open end wrench or adjustable wrench to remove the glowplug. If you had it off before you obviously tightened it WAY too much. If it's factory then just roll your eyes and don't follow their example. It's hard to judge if you don't turn a lot of wrenches but if you are using a Cox flat metal wrench to put on the plug the edges of the wrench should not hurt your hand or leave any indents in your skin with the proper torque. Basically finger tighten it and then put about as much force on it using the wrench as lifting a gallon jug of water. That should do it.