This is a battle that takes place at one of the treasure dungeons known as Hornet Hive. Tiara (girl in the upper right attacking a hornet) would be very useful here because of her strong fire magic.
Treasure Dungeons are the alternate routes of main dungeons. They usually have difficult monsters and hordes of treasure at the very bottom of them (some rewarding, others not so much). This is a good way of strolling away from the main plot just to get some extra cash, experience, and treasure. Expect to find a lot of these.
This originated in Lufia: The legend returns, but I added some stuff to it to make it fresh.