Easy experience and money on Stage 11a (Mazinger Route):
After completing Stage 10, you will be given two choices of routes during the intermission sequences. Select the left route (Battle Dr. Hell in Japan with GoLion/Voltron, Full Metal Panic, and Nadesico as your party). On that Stage, Koji Kabuto will appear in the MazinKaiser (HP 33000) and Baron Ashura in the Guul (HP 31500) as opponents. When that happens, keep MazinKaiser away from the Photon Lab (the only base in the map, as it is one of the requirements that will end the stage in failure) while defeating Baron Ashura in the Guul. Once defeated, the Guul will then revive itself to 9450 HP, but you will still gain experience and money. The Guul will repeatedly revive itself every time it is destroyed until you reduce MazinKaiser's HP to 30% or below. Continue defeating the Guul to gain as much experience and HP as you wish. After, reduce the MazinKaiser's HP to 30% to continue the game and fight Sadak (Commander Yurak) and the Robeasts from Voltron as well as Ashura Mazinger from MazinKaiser.