Heatran @ Leftovers
Ability: Flash Fire
EVs: 252 SpA / 4 Def / 252 Spe
Timid Nature
- Magma Storm
- Earth Power
- Taunt
- Toxic
Heatran checks annoying Steel-types (Ferrothorn, Scizor, Jirachi, and Klefki) and Grass-types (Serperior, Celebi, and Amoonguss). Obviously, this is assuming they don't outspeed Heatran while carrying SE coverage (i.e. HP Ground Serp). Also, Heatran walls non-HP Ground Volcarona.
In addition, Heatran can trap and shit on other annoying bulky mons like Unaware Clefable, Cresselia, and Mew. I know Breloom can use Spore on them and then set up, but that wouldn't always happen due to sleep clause.
In return, Heatran appreciates Breloom's ability to check Water-types (Azu, Slowbro, Quag, etc.), M-Lop, Hippowdon, as well as threatening rain and sand. Mega Diancie supports it to some extent by checking Fighting-types (Breloom, non-scarf Keldeo, non-BPunch M-Medicham) and TankChomp, who pose a threat to Heatran.
Sadly even with Heatran, the core is still weak to ScarfChomp, Iron Tail + fighting coverage Torn-T, Starmie, HP Ground Serp, Manaphy, BPunch M-Medicham, and so on.