Having a faded version of where the attack will aim towards, the 0.5 second before it actually activates, could be a decent indicator.
Downside is this method is less useful for cluttered screens w/ many player attack-skills, so you could have colored arrows instead of faded attack indicators.
It's really helpful to know which enemy will die to random attack, than to wait for it and then kill the stuff that wasn't targeted to avoid overkill wasting dps.