Ok. If I have ranged units, I always focus fire. mainly i will have fiends since i always am undead, so 10-12 fiends and 2 heros attack a hero at once with coil and nova/impale. after i kill the hero, I usualy kill the stronger units or in different cases support units.
Like with mountain giants, if you can take out archers or drydas behind the mg, ignoreing taunt and continueing an attack, you can be very successful.