Heatran @ Air Balloon  
Ability: Flash Fire  
EVs: 244 SpA / 12 SpD / 252 Spe  
Timid Nature  
IVs: 0 Atk  
- Taunt  
- Magma Storm  
- Stealth Rock / Toxic 
- Flash Cannon / Earth Power

Possible Items: Leftovers, Air Balloon, Steelium Z, Groundium Z, Firium Z. 

Role:
Anti lead. Taunt + Trapping the opponent for maximised pleasure. 

Set
Max speed for obvious reasons. The 12 SpD EVs prevent it from getting OHKO'd by a Focus Blast from a Modest Xerneas. The rest of the set is similar to its OU set. Items are whatever you want it to me. Steelium Z to one shot Xerneas. Groundium Z to one shot offensive Groudon-Primal sets. Firium Z for a more reliable fire stab (75% accuracy is meh). 

Calcs:
244 SpA Heatran Tectonic Rage (175 BP) vs. 0 HP / 100 SpD Groudon-Primal: 372-438 (109 - 128.4%) -- guaranteed OHKO
244 SpA Heatran Corkscrew Crash (160 BP) vs. 0 HP / 0 SpD Xerneas: 528-624 (134.3 - 158.7%) -- guaranteed OHKO
244 SpA Heatran Magma Storm vs. 4 HP / 0 SpD Magnezone: 356-420 (126.2 - 148.9%) -- guaranteed OHKO
244 SpA Heatran Earth Power vs. 0 HP / 0 SpD Tapu Koko: 248-294 (88.2 - 104.6%) -- guaranteed OHKO after Stealth Rock
244 SpA Heatran Corkscrew Crash (160 BP) vs. 0 HP / 4 SpD Nidoking: 328-387 (108.2 - 127.7%) -- guaranteed OHKO
244 SpA Heatran Inferno Overdrive (180 BP) vs. 252 HP / 4 SpD Magearna: 518-612 (102.7 - 121.4%) -- guaranteed OHKO (Inherited 150 HP)
244 SpA Heatran Magma Storm vs. 252 HP / 232+ SpD Ferrothorn: 432-508 (85.7 - 100.7%) -- 37.5% chance to OHKO after Leftovers recovery and trapping damage (Inherited 150 HP)
244 SpA Heatran Corkscrew Crash (160 BP) vs. 0 HP / 4 SpD Tyranitar-Mega in Sand: 296-350 (86.8 - 102.6%) -- 93.8% chance to OHKO after Stealth Rock

Defensive Calcs
252+ SpA Xerneas Focus Blast vs. 0 HP / 12 SpD Heatran: 272-322 (84.2 - 99.6%) -- guaranteed 2HKO
252 Atk Tyranitar-Mega Stone Edge vs. 0 HP / 0 Def Heatran: 186-219 (57.5 - 67.8%) -- guaranteed 2HKO
252 SpA Choice Specs Swellow Boomburst vs. 0 HP / 12 SpD Heatran: 182-215 (56.3 - 66.5%) -- guaranteed 2HKO