This core is pretty common and well known but no one has actually posted it in here so I decided that I would.
Hydraslash
Hydreigon @ Life Orb
Ability: Levitate
EVs: 4 HP / 252 SpA / 252 Spe
Modest Nature
- Draco Meteor
- Dark Pulse
- Tailwind/Fire Blast/Earth Power
- Protect
Aegislash @ Leftovers
Ability: Stance Change
EVs: 236 HP / 252 SpA / 20 Spe
Modest Nature
- Shadow Ball
- Flash Cannon
- Substitute/Wide Guard/Shadow Sneak (If running Wide Guard, I prefer using Weakness Policy or Safety Goggles, if running Sneak I use Life Orb)
- King's Shield
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Hydraslash has been a pretty common core since the beginning of XY doubles. It's such an effective core because of the perfect defensive synergy that the two have together...
Aegislash (ignoring abilities):
Weaknesses: Dark, Fire, Ghost, Ground
Resistances:
Bug, Dragon, Fairy, Flying, Grass, Ice, Psychic, Rock, Steel
Immunities: Fighting, Normal, Poison
Hydreigon (ignoring abilities):
Weaknesses: Bug, Dragon,
Fairy, Fighting, Ice
Resistances: Dark, Electric, Fire, Ghost, Grass, Water
Immunities: Psychic
As you can see hydreigon can switch into every single thing that super effectively hits aegislash and vice versa. Hydreigon hits pretty much everything really hard and aegislash is no slouch offensively as well. Playstyle wise, I like to have only one of these out at a time. For example, if my hydreigon is out and I'm predicting a draco meteor onto it, I simply switch in the aegislash etc. I can't really think of anything else that needs to be said except that this core has been super solid for the past two-ish years or so for a reason.
EDIT: I forgot to mention that lando-t can threaten both of these mons. Because of this, I like having a bulky water (suicune or washtom usually) paired with this core to help deal with that.