The general strategy with any Rock Polish set is to come in on something that can be scared away and then set up. Bonsly is a prime example of a Pokemon that can do this with its high Defense stat. Furthermore, the popularity of Choice Scarf Flying-type Pokemon means Bonsly can set up with ease, while only receiving minor injuries.
Believe it or not, because Bonsly lacks Stone Edge, Double-Edge is its best attacking move; its 120 base power trumps Rock Slide's power of 112 after STAB. Due to Bonsly's ability, Rock Head, the recoil from Double-Edge will be suppressed, allowing the Bonsai Pokemon to sweep effectively without losing too much HP. Rock Slide is a secondary attacking move used for its coverage and high 30% flinch rate. Outside of these two moves, Bonsly doesn't have any other powerful options, barring Explosion and Self-Destruct, so Brick Break is used for coverage. With an Adamant nature, Brick Break has a 99.41% chance of 2HKOing standard Ferroseed, so switch-ins are doomed if you predict well. With an Adamant nature, however, a +2 Bonsly is outsped by Snover and Magnemite holding Choice Scarf, two opponents that Bonsly otherwise beats by outspeeding. Because of this, a Jolly nature is an option if your team has trouble with the aforementioned threats. With Jolly, Bonsly has a minuscule chance (less than 3%) of 2HKOing Ferroseed with Brick Break, but it will always outspeed Snover, and it will at least tie with Timid Choice Scarf Magnemite.
If your team has a Steel trapper, you can use Sucker Punch with an Adamant nature in the last slot. This is good for both revenge killing and beating Choice Scarf Gastly before it can use Hidden Power Fighting. Explosion is an option to go out with a bang.
Eviolite is an option to improve Bonsly's already impressive Defense stat and make its Special Defense usable, but your team will need lots of entry hazards to make up for the lost power. Bonsly has access to Dig, which can be useful to hit both Poison- and Steel-types, but it is very easy to work around. Fortunately, Bonsly beats any Flying-type Pokemon not named Mantyke, Wingull, or Ducklett, so it isn't too much of a loss.
Slowpoke is a good teammate to wall Fighting-type Pokemon. Cottonee is another good teammate; it has excellent synergy with Bonsly and can give it time to set up with Encore. Entry hazards, namely Spikes to hit Steel-type Pokemon harder, are good support options, especially if Bonsly has a Jolly nature, as they make a 2HKO on Ferroseed guaranteed.
