For single targets use nova shot at close range.
Crushing blow basically reduces monster hp by 25% from it's current value. And it's effect halved against bosses and halved for ranged attacks. In other words it reduces:
Against normal monster: 25% hp from melee and 12,5% hp from ranged attack.
Against bosses (not only act bosses, but each superunique and/or champion): 12,5% hp from melee and 6,25% hp from ranged attack.
PS: Also effect reduces even more against players and hirelings. Only 10% from melee and 5% from ranged attack.