If it helps, it seems like the weapon hitboxes are pretty faithful to the animation when you attack.
So if you are performing a diagonal swing and want to hit something low down, you want to angle yourself so that the enemy is facing the side where your swing comes to its lowest point. If the lowest point of your swing is on your bottom left you want to face yourself a little to the right and vice versa.

Similar logic can also be applied when using your pickaxe to hit things either precisely or overhead.