In this code, when a missile collides with ImageSprite2 (enemy), if the HP of the mirror (the "mirror" is supposed to reflect the missile but I haven't implemented that yet) is more than 0, the mirror's HP is reduced by 1 (the "MirrorHP ≠ 1 part decides the sound). If the MirrorHP IS 0, the BossHP is reduced by 1. The "missilecomeback" procedure is supposed to disable and hide the missile (it used to return to the player, thus explaining the name), and the "x" input decides what missile is affected.
You will need to debug each of your if conditions 1 by 1, then check the setting for the two variables.
Does other - ImageSprite2?
Is MirrorHP greater than 0 ?
Is MirrorHP not equal to 1 ?
What happens in missilecomeback ? Does that work with the value you provide ?