Excepting for times where you want to focus on an overpowered enemy that can do a lot of damage, consider attacking the enemy that you can destroy the fastest first. Do this before it attacks or at least before it manages can attack twice. Target one enemy at a time and destroy them before moving to the next. You can include area of effect attacks while doing so. During Boss battles, destroy the supporting troops they bring with them then focus on the Boss at the end.