And the denizens of the dungeon will just let you do that, week after week until they're all dead?
Yes? We chained Fireball 5 out of 40 goblins. Now there's fewer goblins. They send 5 goblins for help. Guess what? there are EVEN FEWER goblins now. So now we can go HARDER.
The WHOLE RANGER CLASS in based on the concept that if you ambush and run away the enemy enough while having decent defensive capacity, the enemy will either all die or run away before they take too many losses.