I personally like this as my defensive core as it covers a lot of ground with just the three of them.
Jellicent @ Leftovers
Ability: Water Absorb
Bold
248 HP/ 216 Def/ 44 Speed
Recover
Scald
Toxic
Taunt
Heatran @ Leftovers
Ability: Flash Fire
Calm
252 HP/ 4 Def/ 252 Sp. Def...