Aggron-Mega @ Aggronite  
Ability: Filter  
EVs: 252 HP / 4 Atk / 252 SpD  
Sassy / Careful Nature  
- Curse  
- Heavy Slam / Iron Head  
- Rest  
- Sleep Talk / Toxic / Earthquake / Fire Punch

Role:
Tank, a massive bulky steel tank with 154 Inherited SpD from Ho-oh or Lugia. It functions nicely with both of these mons as well! 

Set:
Max HP and max SpD with a beneficial nature allows Aggron to live a huge amount of special attacks (see calcs).  And with its 230 base defense and filter, its bulky on both sides. 
Curse and heavy slam are its main selling point. Dealing loads of damage to anything that isnt quad resisted by it. Iron head can be used to finish off Xerneas, because Heavy Slam has 40 power vs it. A resttalk set can be used but sleep talk can be traded in for toxic for stall and Earthquake for Magnet Pull mons and Primal Groudon and more heavy rock types like Tyranitar-Mega. Funnily enough, the 4 EVs in attack are JUST enough to guarantee a OHKO vs Swellow. Fire punch for Skarmory, Scizor and Ferrothorn. 

Defensive Calcs:
+2 252+ SpA Xerneas Focus Blast vs. 252 HP / 252+ SpD Filter Aggron-Mega: 229-271 (62.9 - 74.4%) -- guaranteed 2HKO
212+ SpA Groudon-Primal Overheat vs. 252 HP / 252+ SpD Filter Aggron-Mega in Harsh Sunshine: 303-357 (83.2 - 98%) -- guaranteed 2HKO
252 SpA Choice Specs Swellow Heat Wave vs. 252 HP / 252+ SpD Filter Aggron-Mega: 139-165 (38.1 - 45.3%) -- guaranteed 3HKO
+2 252 Atk Tyranitar-Mega Fire Punch vs. 252 HP / 0 Def Filter Aggron-Mega: 139-165 (38.1 - 45.3%) -- guaranteed 3HKO
200+ SpA Kyogre-Primal Origin Pulse vs. 252 HP / 252+ SpD Aggron-Mega in Heavy Rain: 195-231 (53.5 - 63.4%) -- guaranteed 2HKO
252 SpA Tapu Koko Thunderbolt vs. 252 HP / 252+ SpD Aggron-Mega in Electric Terrain: 133-157 (36.5 - 43.1%) -- guaranteed 3HKO (Inherited 154 SpA)
216 SpA Mewtwo-Mega-Y Fire Blast vs. 252 HP / 252+ SpD Filter Aggron-Mega: 127-150 (34.8 - 41.2%) -- guaranteed 3HKO
252+ Atk Thick Club Marowak-Alola Flare Blitz vs. 252 HP / 0 Def Filter Aggron-Mega: 343-405 (94.2 - 111.2%) -- 68.8% chance to OHKO (inherited 150 Attack, make sure you DONT have an unboosted aggron vs this thing)
252 SpA Life Orb Sheer Force Nidoking Earth Power vs. 252 HP / 252+ SpD Filter Aggron-Mega: 224-265 (61.5 - 72.8%) -- guaranteed 2HKO

Offensive Calcs:
+1 4 Atk Aggron-Mega Heavy Slam (120 BP) vs. 0 HP / 0 Def Nidoking: 322-381 (106.2 - 125.7%) -- guaranteed OHKO
4 Atk Aggron-Mega Heavy Slam (120 BP) vs. 0 HP / 4 Def Swellow: 261-307 (100 - 117.6%) -- guaranteed OHKO
4 Atk Aggron-Mega Heavy Slam (120 BP) vs. 0 HP / 0 Def Diancie-Mega: 640-756 (145.1 - 171.4%) -- guaranteed OHKO (inherited 150 HP)
+2 4 Atk Aggron-Mega Fire Punch vs. 252 HP / 4 Def Ferrothorn: 456-540 (90.4 - 107.1%) -- 43.8% chance to OHKO after Leftovers recovery (Inherited 150 HP)
+3 4 Atk Aggron-Mega Earthquake vs. 0 HP / 0 Def Groudon-Primal: 318-376 (93.2 - 110.2%) -- 62.5% chance to OHKO