when a banshee appears, it will shoot orbs out from its mouth which will spawn little black monsters. kill these quick because if you don't the banshee will take a some of your health and transfer it to itself. when you have killed the last little monster, the banshee will bend over. Use this opportunity to shoot it with a ranged weapon, preferably a repeating one. If you fail to kill it in time, it will spawn more little monsters and you must repeat the process.