Mega-ampharos + Excadril + Moltres makes a pretty good defensive core. Mega-Ampharos's unique set of useful resistances outside makes it a very good switch-in to common attacks running around in the metagame. Along with that good bulk makes M-Ampharos one of the best tanks around, especially when it hits hard even without investment. Moltres can switch into common into Ground and Fairy attacks targeted at Ampharos, as well as being tanky enough to take most Dragon and Ice special attacks. Excadril rounds off the core by resisting Rock and Electric attacks targeted at Moltres, Dragon attacks, as well as spinning away rocks easily.
The biggest problem for this core is probably Charizard X if it comes in on Moltres and get a DD up. For that reason, Azumarill also can fit in the team for being one the its best checks.
Ampharos @ Ampharosite
Ability: Static
EVs: 192 SDef / 252 HP / 44 SAtk / 20 Spd
Modest Nature
- Volt Switch
- Dragon Pulse
- Rest
- Sleep Talk
Excadrill @ Leftovers
Ability: Mold Breaker
EVs: 96 Atk / 160 HP / 252 SDef
Adamant Nature
- Earthquake
- Shadow Claw
- Rapid Spin
- Swords Dance
Moltres @ Leftovers
Ability: Pressure
EVs: 116 Spd / 248 HP / 144 SDef
Calm Nature
- Substitute
- Roost
- Will-O-Wisp/Toxic
- Flamethrower
The biggest problem for this core is probably Charizard X if it comes in on Moltres and get a DD up. For that reason, Azumarill also can fit in the team for being one the its best checks.