The aim of this set is to support your team with Stealth Rock and Rapid Spin. As Ghost-types will be constantly switching in to block Baltoy's Rapid Spin, Shadow Ball is a necessary weapon, hitting Gastly and Drifloon very hard and making Duskull think twice about switching in. The final move should be selected depending on what you want Baltoy to hit harder. Earth Power gets STAB and demolishes Croagunk and Stunky, two Pokemon that would normally threaten vulnerable Psychic-types like Baltoy with Sucker Punch or Pursuit, easily OHKOing Croagunk and potentially doing the same to Stunky. On the other hand, Explosion destroys Munchlax and Mantyke. Oran Berry will grant Baltoy extra longevity, giving it more time to support the team. A Bold nature is preferred in order to improve Baltoy's lower Defense stat. However, if Explosion is being used on this set, a Relaxed nature is preferred to prevent Baltoy from losing a point in Attack. The EVs maximize Baltoy's physical bulk, and the remaining EVs give an extra point apiece to Baltoy's Special Attack and Special Defense stats.
Psychic would give Baltoy a secondary STAB outside of Earth Power, but Shadow Ball is generally the better option because it hits all Ghost-types, not just Gastly, for super effective damage. Reflect can be used in the last slot to reduce the damage taken from Pursuit users, but Earth Power and Explosion are preferred since they can remove said Pursuit users directly.
Mantyke, Houndour, Snover, and Magnemite are all great partners for this set, as they also have good synergy with Baltoy. For example, Mantyke benefits greatly from Baltoy's talents, as Baltoy can clear Stealth Rock for Mantyke while also easily taking the Electric- and Rock-type attacks that threaten. Houndour can take Ice-, Ghost-, and Dark-type attacks aimed at Baltoy, while Baltoy is immune to the Ground-type attacks and resists the Fighting-type attacks that plague Houndour. Unfortunately, Baltoy and Houndour share a weakness to Water-type attacks. In Snover's case, Baltoy resists Snover's weaknesses to Fighting- and Rock-type attacks, while Snover resists Baltoy's weaknesses to Water- and Grass-type attacks. Magnemite is probably the best partner to Baltoy, as they have near-perfect synergy. Magnemite can take any Grass-, Ghost-, or Dark-type attack, while Baltoy resists Fighting-type attacks and is immune to Ground-type attacks. More importantly, Magnemite traps Bronzor, which beats this set with ease (assuming Bronzor carries Hidden Power Ice). Poison-types such as Croagunk and Stunky are also good teammates because they can take Grass-type attacks for Baltoy while Baltoy is immune to the Ground-type attacks that they fear. Finally, Normal-types such as Munchlax are immune to Ghost-type attacks, and Baltoy resists Fighting-type attacks.
Pokemon that are adept at removing Ghost-types help Baltoy exponentially, as Ghost-types can block Rapid Spin and threaten Baltoy with their STAB moves. Stunky, Munchlax, and other Pursuit users can trap and KO most Ghost-types, which greatly increases Baltoy's usefulness.