Lock on while a nice skill/feature, has a lot of problems being used in practice. In order to lock on you have to quickly tap the lock on button, then tap the monster part, while the monster is moving, and most of the time the lock on targets the wrong part or doesn’t even register your touch. This problem gets worse with the distance sensitivity of some of the lock on targets.
So instead of locking on by clicking on the monster, why not have lock on be done through the button, in the form of a drop down menu. You click the lock on button, and the button expands into a dropdown menu with little icons representing each part, then you click the part you want to lock on and the part is now targeted. This would help so much with fights as a miss input on lock on (which is very common) can sometimes require restarting the fight.