Shiftry is better. It takes down Tate and Liza super effectively, along with Wallace, and Phoebe. Shroomish can only be found in Petalburg Woods, so you would have to do a whole lot of grinding if you didn't already have one. You also already have a Fighting type in the form of Combusken, so Breloom would overlap types. Unfortunately, Shiftry can only be obtained in Omega Ruby without trading, so if you're playing Alpha Sapphire, you'll have to use Breloom. Use this set:
Shiftry @ anything
Ability: doesn't matter
- Extrasensory
- Energy Ball
- Dark Pulse
- Shadow Ball / Brick Break / Strength / anything else
If you decide to use Breloom:
Breloom @ anything
Ability: Doesn't matter
- Mach Punch / Brick Break
- Seed Bomb
- Stone Edge / Rock Slide
- Return
Hope I helped!