Here we have in Figure A, a Dark Beast in a non favorable position. In this position your cannon only fires single shots. Here is where I will attempt to school you on the intracacies of lining the npc up. My choice to educate using arguably one of the most difficult npcs to manipulate is because if you can master this, you can certainly do all of the less difficult npcs.
The
GREEN arrow indicates where you want the NPC.
The
YELLOW arrow indicates what you are waiting for. The target Dark Beast will NOT move under that present model. You have to wait patiently for conditions to be right. In this case, for the Dark Beast that is blocking to move towards the direction the arrow indicates. That will then open up the possibility of manipulating the target into the desired spot.
The
ORANGE arrow indicates the square you want to move to. This is only a temporary move. You will want to move immediately back into the target range or two things could happen. 1) The target npc could start firing its magic attack or 2) Another close by npc will start attacking you.
Always move in smoothe linear motions along a solid axis. More on that later.

In figure A-2 you see that the Dark beast has moved into the desired position and you are getting three chances for damage each round. I snapped a screenshot of 3 zeros on purpose as to illustrate that simply because you CAN hit three times per round does not mean you are going to each round. Here at the Dark Beasts it's far more common to miss than to hit as they have notoriously high defense.

Here in figure A-3 I show in
ORANGE the correct axis to move in and still remain in battle. Remaining within it's melee distance is imperative to keep it from moving. One square outside that range will manipulate it to move towards you. Two squares MAY provoke it to start using it's magic attack.
In this figure I also show where reloading is feasible. The
ORANGE check marks are the best spots for reloading while still maintaining the melee combat lock.

It is the responsibility of intellectuals to speak the truth and expose lies.
~Noam Chomsky