A reducer, as I understand it, is indeed a 'brutal challenge' against a talented player. It describes the first challenge of the game, which is intended to show the player who is boss. After which, the game proceeds more or less within the rules (assuming the 'reducing' player has not been sent off ...)
A reducer is a brutal challenge on a talented member of the opposing side. The term was first coined by bumbling pundit and casual racist Ron Atkinson.
"It's the kind of thing you say to your most competitive defender when he's up against someone skillful: 'Put the reducer on him.'" - Atkinson told popular London Underground seat cushion Metro.
Although the term reducer is relatively new to the language of football, the concept of fouling the other side's best player is timeless. Reducers are often performed by no-nonsense players.
Greats such as Pelé and Diego Maradona spent much of their career being pursued by a gang of less gifted players with evil intent. Initially this lynch mob settled for a few stiff kicks in the groin, later on it became common for at least one to be armed.