Low Speed and Fire typing hurt Flareon in standard play, but it's a strong UU Pokemon who hits hard on both fronts. Strike fast and don't let the opponent know what exactly happened; just watch out for those Water- and Rock-types.
It has decent HP for a Fire-type and the traditional low Defense, but trades a Fire-type's traditional high Speed (it'd be awesome at even 278 Speed) for a superb 318 Special and gigantic 358 Attack. In short, Flareon is a powerhouse.
If you're playing on Stadium, the best strategy is to use Focus Energy and then Fire Blast away until Flareon dies or the opponent loses. Hopefully it's the latter. Unfortunately, Focus Energy doesn't work properly in any other environment. In that case, just use Quick Attack; it takes advantage of Flareon's high Attack, and allows Flareon to surprise faster Pokemon.
Mimic is cool for getting a new move, like Earthquake. Substitute is nice for protection. Leer can lower a switch-ins Defense so you can hopefully OHKO or 2HKO it. Sand-Attack is risky, but allows Flareon to take down much tougher foes with some luck and works well with Substitute and Body Slam paralysis.
Checks and Counters
In OU play, the obvious counters are any Water- or Rock-type and the ever present and frightening Tauros. If Flareon is being used in UU play, the list of counters remains similar. Again, any Water- or Rock-type can immediately shut it down. Omastar and Kabutops are surefire ways of countering Flareon in UU since they are 4x resistant to Fire-type attacks and 2x resistant to Normal-type attacks, which make up the majority of Flareon's movesets.