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.
Adamant
Leftovers
200 HP / 64 ATK / 244 DEF
U-turn
Earthquake
Stone Edge
Stealth Rock
Modest
Leftovers
184 HP / 4 Def / 144 SpA / 176 Spe
Pain Split
Will-O-Wisp
Hydro Pump
Volt Switch
Careful
Leftovers
252 HP / 252 SP. Def / 4 Def
Wish
Protect
Iron Head
Body Slam