Heatran (M) @ Air Balloon Trait: Flash Fire
EVs: 56 HP / 252 SAtk / 200 Spd
Timid Nature (+Spd, -Atk)
- Fire Blast
- Earth Power
- Hidden Power [Ice]
- Roar
Jellicent (M) @ Leftovers Trait: Water Absorb
EVs: 248 HP / 36 Def / 136 SDef / 88 Spd
Calm Nature (+SDef, -Atk)
- Surf
- Recover
- Will-O-Wisp
- Taunt
This core forms good defensive synergy for most teams, and protects most teams from rain-boosted Water attacks, and sun-boosted Fire attacks. This core is good for teams that have problems with both sun and rain.
I am using Roar on Heatran for the sheer surprise factor, and also because without it, Heatran cannot actually bypass Volcarona. Roar is also useful for other situations. For example, Fighting-types that try to use Heatran as setup fodder are forced to take more hazard damage and/or to switch-in and take more heavy damage from STAB Fire Blast. If Heatran has its balloon poped and Dugtrio is still alive on the enemy's team, you can use Roar to facilitate prediction. Fire Blast is because of STAB, Earth Power for opposing Heatran and HP Ice for Dragons.
Jellicent is there because it has amazing defensive synergy with Heatran. Thanks to Air Balloon, your opponent will more often than not, attempt to hit Heatran with Fighting- or Water-type attacks. However, Jellicent is immune to both of these types, so as long as Heatran can mantain its Air Balloon, it will be always defended by Jellicent. Not to mention that Jellicent can scare most Ground-types anyway, and Heatran can even defeat some Ground-types by itself.
Heatran protects Jellicent from its worst enemies, Grass-types, while Jellicent walls the Fighting-types and Water-types that would otherwise defeat Heatran.
However, there are three Pokémon that can, unfortunately, defeat this core: Rotom-W (if it carries Thunderbolt), Breloom (if it manages to outspeed Heatran or hit it with Mach Punch), and Hydreigon (if it has Dark Pulse). The former two can be countered by Celebi, while the latter has no counters, but is very easy to check and revenge kill.
This core is amazing to those that want checks to both rain teams and sun teams. Jellicent and Heatran's utility doesn't end here. Jellicent can shut down many walls with a combination of burn and Taunt, while Heatran checks Dragonite and Salamence. The two also resists almost all types thogether. The only three types that they miss out are Electric, Rock, and Ground (but Heatran is immune to Ground with Air Balloon).