If a giant bat hits an enemy on the ground, it will cause only a little damage, but it will hook its claws into a humanoid's clothing, clutching tightly. As the bat will bat its wings against attempts to pull it free, it can only be removed if killed or made unconscious. While attached, the bat will continue to attack its victim. The victim, meanwhile, will be -2 to hit the bats, because they will hold onto the back of a defender or onto a defender's arms. Meanwhile, any attack against the bat in which the defender (or anyone else) rolls a 2, 3 or 4 will cause attack to the defender's self.
Missile attacks against bats are always possible, though it should be noted that bat attacks always occur at night. See Illumination.