Yeah, Garchomp's playstyle changes quite a bit imo after Mega Evo. It goes from a hard-hitting physical attacker that trolls the common base 100 Speed tier to a super strong wallbreaker that trolls one of the best offensive pivots from BW OU, Landorous-T. When Mega, Garchomp doesn't have the Speed needed to sweep, but it can certainly blow up some holes in the opposing team. Here's a set to think about imo:
Mega Mixed Attacker
(Mega) Garchomp @ Garchompite
Trait: (Rough Skin) Sand Force
EVs: 240 Spe / whatever EVs that optimize offenses
Nature: Naughty / Naive
- Earthquake
- Draco Meteor
- Stone Edge / Poison Jab
- Fire Blast
So, yeah, basically, you come in and start doing serious damage to stuff. The best part is that you can keep Garchomp in its normal forme until you either need Draco or Fire Blast or when you are outsped but can utilize the extra bulk gained through Mega Evo and survive the oncoming attack. Earthquake is the reliable physical STAB and it smashes every Fairy-type not named Togekiss p. hard. Draco Meteor is your other STAB and it smashes most EQ-immune Pokemon. The only Pokemon that can easily take this combination is Togekiss, so your 3rd move is almost exlusively for it. Stone Edge is more specialized and gets a stronger hit on Togekiss while getting better general coverage. Poison Jab is an alternative to nail all Fairy-types bar Mawile, who is ruined by EQ anyway. Fire Blast rounds out the coverage to handle physically defensive Steels that can take a Draco Meteor to the face (Ferrothorn and Skarmory being the big two) The Speed EVs let Garchomp outspeed base 100s before activating Mega Evolution while it still allows Garchomp to beat out Lucario (make sure it hasn't Mega Evolved!) and other base 90s after Mega Evo.