Moves
Stealth Rock can offer chip damage to foes on the switch, break Focus Sashes, and severely injure some Pokemon upon switching in, such as Ninjask and Articuno. Toxic is Bastiodon's only way to inflict damage, and it can be very effective in either breaking down walls or weakening a foe to the point where a teammate can revenge kill it. Roar is very useful for phazing Pokemon that are attempting to set up as well as for switching the active foe out, forcing the opponent to send in a more favorable match up. Magic Coat allows Bastiodon to bounce back status moves, such as Will-O-Wisp and Taunt.
Set Details
Maximum investment in Special Defense and HP with a Careful nature allows Bastiodon to take on dangerous threats such as Ninetales, Zebstrika, Rotom-F, Tangela, Roselia, and other special attackers. A physically defensive set with a Bold nature could be used, but it has more special weaknesses, while investment in Defense doesn't help it as much. Leftovers gives Bastiodon a bit of health back each turn, which is important, as it doesn't receive a recovery move. Sturdy allows Bastiodon to survive a singular hit from full health, which can be key in situations when it is trying to get Stealth Rock up or when it needs to Roar out a Pokemon that is trying to sweep.
Usage Tips
Just because Bastiodon has Stealth Rock doesn't mean it will always be the Pokemon you will lead with. Don't solely rely on Bastiodon to take on a lot of threats to your team, because it can get weakened quickly. Because it lacks a recovery move, you shouldn't consistently switch it into attacks. In addition, Bastiodon needs to fear coverage moves on foes such as Mr. Mime and Misdreavus. If you can, attempt to preserve Sturdy for late-game as a last resort against setup sweepers. Use Magic Coat only when you are certain the foe is going to use a status move. Attempt to set up Stealth Rock early-game to allow it to have the largest impact on the opposing team as possible, and Toxic can be spammed early-game to weaken foes as well.
Team Options
Bastiodon works best on balanced or stall teams in which its teammates can counteract foes that beat it; these teams also appreciate Bastiodon's entry hazard support. Bastiodon works well with Pokemon that can counteract Fighting- and Ground-type moves, such as Clefairy, Arbok, and Jumpluff. Bastiodon also appreciates Pokemon that can deal with Steel-types, such as Monferno, Ninetales, and Rapidash. Ghost-type Pokemon such as Dusknoir, Gourgeist-XL, and Misdreavus can block incoming Rapid Spins and keep Bastiodon's Stealth Rock up; their immunity to Fighting-type moves is also beneficial. Bulky Grass-types such as Roselia, Tangela, and Gourgeist-XL can take both Water- and Ground-type moves. Wish passers such as Audino allow Bastiodon to recover HP and stay on the field longer. Spikes setters such as Roselia and Quilladin can set up Spikes or Toxic Spikes, and they allow Bastiodon to get more damage on a foe when using Roar.
Pawniard and Purugly, while both sharing Bastiodon's Fighting-type weakness, are very good partners, as they can discourage the use of Defog from the foe and maintain Bastiodon's entry hazard with Defiant. In addition, Pokemon who use Stealth Rock to their advantage, such as Mr. Mime, Sawsbuck, Arbok, Kingler, Articuno, and Ursaring, appreciate Bastiodon on their team to deal with the opponent's team better.