Rotom-Wash @ Leftovers
Ability: Levitate
EVs: 248 HP / 240 Def / 20 SpD
Bold Nature
- Volt Switch
- Hydro Pump
- Will-O-Wisp
- Pain Split/Thunder Wave

Heatran @ Leftovers/Chople Berry/Shuca Berry
Ability: Flash Fire
EVs: 252 HP / 4 SpD / 252 Spe
Timid Nature
- Taunt/Roar
- Toxic
- Lava Plume/Magma Storm
- Stealth Rock/Earth Power/Protect

Venusaur @ Venusaurite
Ability: Chlorophyll
EVs: 232 HP / 180 Def / 80 SpD / 16 Spe
Bold Nature
- Giga Drain
- Sludge Bomb
- Synthesis
- Hidden Power Fire/Knock Off/Leech Seed

Basically, these two have really good synergy together, and wall a large portion of the metagame. Stuff that threatens Heatran, like Azumarill and Keldeo are easily walled by Venusaur. Stuff that threatens Venusaur like Talonflame and Latios are walled by Heatran (bar EQ Latios which isn't exactly common). Heatran is also a very effective SR setter, phazer, and is just a pain in the butt do deal with. So is Venusaur. They are both really bulky, so they can comfortably take lots of hits, and Venusaur gets recovery with Synthesis. These two are pretty much the main staples on stall. However, what makes them really effective in the ORAS metagame is that they wall most of the top tier threats together. Pokemon like Latios, Azumarill, Zard X without EQ, Clefable, Keldeo, Talonflame, Ferrothorn, Slowbro etc are all handled well by this core.
There are still lots of ways to beat this core though. Landorus can easily crush this core with Sheer Force boosted Earth Power and Psychic, Mega Pinsir literally eats this core for breakfast with it's high powered STAB moves, and takes care of Heatran easily with either Close Combat or Earthquake. Mega Metagross is also annoying to deal with, but if Heatran is healthy enough it can deal with Hammer Arm variants. However, you can also play around Mega Metagross, for example, you could send in Venusaur, and on the Zen Headbutt go into Heatran. Then switch back into Venusaur on the Hammer Arm and then back into Heatran on the Zen headbutt. Now you outspeed and can hurt it with Lava Plume. This is really risky though, since your opponent can outplay you and predict your switches. Mega Gallade is also really annoying, as it can literally beat this core just because of it's offensive typing. Same thing for Mega Gardevoir, however, it needs to rely on the shaky accuracy of Focus Blast to beat Heatran. Kyu-B and SD Mega Scizor (with Superpower) also beat this core. Too lazy to list all the threats to this core since no core is perfect.
Anyways, you guys get the idea. Venusaur and Heatran handle the majority of the top tier threats, and are staples on stall. Something else that I forgot to mention is that they form 2/3 of an FWG core, so feel free to add something like ScarfKeld since it can check Landorus and severely damage Mega Metagross, Mega Gallade, and Mega Gardevoir.Rotom-Wash provides solid synergy as well as coverage.