It's annoying that some creatures (especially the flying ones) are hard to select for primary weapon attack by left-click, this is also the case when multiple characters are near each other. This proves no problem when you want to perform a skill attack/spell/charm : you can select the spell, hover your mouse over the icons on top of the screen and click on the one you want to perform it on. It's a good thing the characters also light up when you do this, so you allways now which one you're targetting exactly.
But just left-clicking on these icons gives no reaction at all, so my suggestion is make left-clicking the instruction to perform a primary weapon attack on the selected target.