Pretty much everything has been said already about landing a successful combo
Might add that the unit SMARTLINK will increase your ranged accuracy on non ranger classes. (20 past base if if I'm not mistaking)
Also a normal attack will land more often than a heavy attack, which will still land more often than most special attacks. (there are exceptions)
A good combo in ultimate when you often miss, is [normal, hard, hard], [normal, normal, hard] or [normal, hard, special] (again there are exceptions)
The only way to increase your ATA is by equipment/weapons, dex on your mag and leveling up.