Use the right size wrench. Use a cheater bar on a socket or use the 2 wrench trick to increase leverage. Be sure to not drain fluid until you have the fill plug out.
Use some Croil on the plug and give it 10-15 minutes to soak into the threads. Absolute worst case bring it to a shop before you do any real damage. If you have an aluminum case heat won't do much and could hurt any plastic or burn able stuff.