As you venture further into the game you will encounter powerful Mogas. The first areas you will venture to will have fairly easy Mogas. You can fight the easy Mogas over and over again to level up all of your best Mogas. As you level up your Mogas will gain more powerful skills. You'll eventually be able to defeat easy Mogas with one hit.
When you click on an area, if it says "Easy" the Mogas you will encounter are the same level or lower than your Mogas. "Medium" areas will have Mogas one or two levels higher than your Mogas. "Hard" areas will have Mogas several levels higher than your Mogas.