Me and nilly tried this, although it works, it will not be 100% accurate on moving mobs.
Spell projectiles aren't the same as bullets being fired from the player.
You send a USEITEM Packet to the server indicating you are using a spell, and the server sends you back SHOOT packets to display the projectiles from the location you sent in the USEITEM packet for the spell. If the mob is fast-moving, the small amount of lag you receive before the SHOOT arrives will mean that it will you will be aiming at the last position the mob was in.
Now there are also issues for mobs who stand still (like Limon in Sprite Worlds), depending on the position of the mob, sometimes none of the spells will hit at all. I believe this could be edited to avoid this, but I do not have the time mess with it at the moment.