there are a lot of things you can do like casting hero spells, focus fire, make units attack the unit they counter, casting unit spells, retreating or deleting damaged units.

now my question is in which order of priority should you do that when you don't have time to do it all

U have time for it all, u just need to be quick enough

Cool thread

Useless thread

I guess the most important are:
1st. Retreating damaged unit
2nd. Casting heroes spell

But as cr3at1ve said, you just have to be quick enough to do all at the same time ^_^!

With large(r) armys it is impossible. No human can be fast enough and pros dont micro every unit perfectly. Ofc, in 50 vs 50 bear mirror it can be possible but not in 70vs70 orc vs hum.

The biggest priority should be on your hero at anytime. Losing your hero in late game fights usually ends the game or gives huge time/money advantages to your enemy.

So, to answer your question: Everything hero related comes first, with dh you burn whenever you can, with am you summon we/blizzard when cooldown runs out and so on.