After you leave Cannon Village and cross the poison swamp, there is a place surrounded by mountains called Evil Hideout. You will find two gatekeepers blocking your way. There are two paths you can take to reach the top floor, and different monsters will appear on each one. It's a good idea to choose the path with monsters that your Denpa Men have elemental advantage against.

You do not need to take both paths to get the special item awaiting you on the top floor. When you reach the top, push the button to collect your item. So select your Denpa Men carefully to explore Evil Hideout!