Though the Insane Cultist has the ability to call other Insane Cultists, its attacks don't do too much damage. This is a good way to gain a lot of experience and money: don't kill the last Insane Cultist in your battle until he has called another one to battle. As such, you can defeat as many as you want all in one battle, which translates to a massive experience boost. Bashing is sufficient to kill them.