This trope is sometimes combined with Plot Tailored to the Party. Multiple characters who attack a specific enemy may get their asses handed to them not necessarily because they're weak but because they're not the suited people to fight that specific foe. A token enemy can easily overwhelm a group of heroes, but a one-on-one will be much harder fight.
Miyamoto Musashi said in the Book of Five Rings that "one man can beat ten men. Just as one man can beat ten, so a hundred men can beat a thousand, and a thousand men can beat ten thousand. In my strategy, one man is the same as ten thousand"