Bonsly is by no means bad; it just has no place in Little Cup. One might say that it is completely outclassed by Geodude, which has higher Speed, access to Stone Edge and Earthquake (both of which Bonsly lacks), and a better physical movepool, with powerful moves such as Superpower. Bonsly's pure Rock typing, however, leaves it less vulnerable to the many weaknesses given by Geodude's Rock / Ground typing. Bonsly's Attack is high enough to attempt a sweep, and its even higher Defense gives it opportunities to set up, traits other pure Rock-type Pokemon such as Nosepass and Roggenrola lack. Bonsly even has Rock Head, which makes Double-Edge a good attacking move. Ultimately, if your team supports Bonsly enough, it can wreak havoc.
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 Selfdestruct, 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.Team Options & Additional Comments >>>
Choice Band and Eviolite sets are decent item choices, depending on what set Bonsly is running. A Curse set in sandstorm with Curse, Rest, Sleep Talk, and Rock Slide not only makes up for Bonsly's mediocre Special Defense, but boosts its already excellent Defense and Attack stats. Stealth Rock is a good support move, but Ferroseed and Lileep outclass Bonsly in terms of support. Using Sturdy and Flail together is an interesting strategy, as Sturdy will leave Bonsly at 1 HP, making Flail do massive amounts of damage. Lastly, Magic Coat, Toxic, and Copycat are very obscure, situational moves that might have some use if you're lucky.
Bonsly receives Rattled from the Dream World. Rattled is a pretty cool ability to help Bonsly's bad Speed, raising it one stage if hit by a Bug-, Ghost-, or Dark-type move; if you're not using Double-Edge, it is the better ability.
Checks and Counters
Slowpoke takes Bonsly's hits easily and can OHKO it with Surf. Choice Scarf Pawniard and Choice Scarf or bulky variants of Chinchou OHKO Bonsly with super effective STAB moves. Any Fighting-type is a good check to Bonsly, as they resist Rock Slide and can OHKO with almost any STAB move. Grass-types share the same trait, though they will have a hard time switching in. Lastly, entry hazards greatly increase the amount of residual damage Bonsly takes, meaning it can easily faint from Life Orb or Double-Edge recoil, depending on its ability.