Aside from various penetrating oils (Aero Kroil, or PB Blaster), For really stubborn pieces, I use a pencil sized flame in a torch, and heat the center of the plug, just to expand it, allow to cool and then hit the plug with a cooling spray. There are several sprays which will go down to -60*F. Normally one would heat the outside, and cool the plug. You don't want to harm the fiberglass!
Usually a crescent wrench or box wrench is best. Be sure it is really snug.If using a socket, use a square head socket, not a 12 point--it will not have enough surface contact. Even a Harbor freight for $4 will work here. Also you can file the plug to one size smaller head and then use an impact wrench to break it free. Make sure it is snug. I carry a battery powered impact driver in my tow vehicle or RV.
After all of this if you are taking out the entire fitting, then over drill the screw holes, undercut the core and fill with thickened epoxy as you should with any core penetration. Also under cut the area around the fitting, and also seal the core with thickened epoxy.
At the very least, get a new bronze plug, and be sure and wrap the threads with teflon tape.