A good defensive core I have been seeing a lot more of lately is Gligar and Umbreon. As they both cover most of the others weaknesses barring certain pokemon like hp ice mienshao.
Gligar @ Eviloite
Trait:Immunity
-Earthquake
-Stealth Rocks
-U-Turn/Toxic
-Roost
Umbreon @ Leftovers
-Wish...