Isaac, 

this is a fixed expression ‘ en prendre un coup ‘ and such expressions are often/usually best just accepted as a whole. They don’t always support efforts at deconstruction word by word, or for grammatical purity ! 

‘en’ is not infrequently one of the words that is found in expressions without an obvious explicable purpose 

https://www.wordreference.com/fren/en%20prendre%20un%20coup

https://www.lawlessfrench.com/expressions/adverbial-en-expressions/