War bat
The War Bat is a large bat found in a dark cave, which can be entered through the tombstone found north of Brightleaf. To enter the dungeon, you need a Mysterious orb which can be obtained from either Bats or Forest bats.
Getting there
To make your way to the boss room, you will first need to obtain a Mysterious orb. If you want to gain experience, Forest bats are a good resource to get a Mysterious orb. However, if your only goal is to get a Mysterious orb as fast as possible, kill the regular Bats as the speed at which you kill them is much higher. It should, on average, take no longer than 5 minutes to obtain an orb.