Note: Bold words are suggested things. Bold and Italic words are non-optional and heavily suggested things.
Defensive Wall/Multi Battle Partner
Bastiodon @Sitrus Berry/Leftovers
EVs: 4 Defense, 252 Sp. Defense, 252 HP
Iron Defense/Toxic: The former is for upgrading its already outstanding Defense, while the latter is a normal stalling and defensive move
Protect/Wide Guard: For critical situations. The latter protects you and your teammate from Surf, Rock Slide and others
Ancient Power/Rock Slide: STAB, the former's Omni Boost helps a lot on defenses, while the latter helps with some flinching and average damage
Flash Cannon/Metal Burst: STAB, the former, when used in a double battle may lower Sp. Defense for teammate to finish, while the latter can be used to counter the opposing Pokémon's attack and making the enemy switch in to another Pokémon
Hope this helped! Comment if I made any mistakes so I can fix them.