Here's a defensive core I've been trying out recently. It's really effective against the meta, with Landorus-T handling physical threats like Terrakion, Garchomp, Breloom, etc. well, while also proving SR and Uturning abilites. Rotom helps check what Land can't, like bulky waters, Mamoswine, etc. Jirachi helps sponge Dracos and Hurricanes, while also providing Wish and paralysis support.
200 HP / 64 ATK / 244 DEF
184 HP / 4 Def / 144 SpA / 176 Spe
252 HP / 252 SP. Def / 4 Def