Like they have said, Flareon's goal is to discard pokemon and get them in the discard to power up Vengance. The pokemon you choose to run should either be ones to aid in the decks consistency (Electrode), survivability (Mr. Mime), or be able to put themselves in the discard without needing external support (Audino, Yamask/Cofagrigus, Ditto if under Cofagrigus), along with Flareon for a main attacker (And leafeon if you want to have a fighting chance with Keldeo).
Cofagrigus is a necessary evil, as it is the easiest pokemon to get into the discard, is a stage 1 so you can run more pokemon, and puts damage on the field for earlier damage and easier knockouts (Makes Kyurem a decent target T2 against Plasma Basics, makes EXs easily achievable in Killing T3, etc), and I happen to run Life Dew as my Ace Spec to have a 1-off free SFU or a free Flareon to attack with, but mostly the former as if played right your opponent will never have the chance to scrapper it away (Do not attempt to fit Recycle in this deck to reuse the Dew.....trust me, it's not worth it)