The Clue
Plan foilers, perhaps (7 letter answer)
The Answer
NEMESES
Tags
Notes
The answer NEMESES is uncommon, appearing about once every 500 puzzles.
Related Clues
Avengers
Banes
Waterloos
Agents of retribution
Archenemies, maybe
Formidable foes
Formidable opponents
Inflicters of retribution
Superheroes, to villains
They're unbeatable
They're unconquerable
Tough opponents
Unbeatable opponents
Batman and Robin, to Catwoman
Batman and Superman, to villains
They give comeuppances
Thorns in one's side
Those imposing retribution
Rivals to who one always loses
They can't be beat
Definition
NEMESIS as a noun:
1. (Greek mythology) the goddess of divine retribution and vengeance
2. (bane, curse, scourge, nemesis) = something causes misery or death; "the bane of my life"