How does the Dwarf ranger action resolve in case many dwarf units are destroyed at the same time? After I attacked a DE opponent, he played a tactic that destroys all units which participated in battle. All units leave play at the same time, does the Dwarf Ranger ability still trigger unit per unit and let me assign 1 damage per destroyed unit? Thanks!

As you said, Ranger shoots once per destroyed dwarf unit. This is vital part of Reclaim the Fallen combo.