Moves
Swords Dance is mandatory, as it boosts Treecko's otherwise mediocre Attack stat to threatening levels and allows it to break past most of the metagame with its attacks. Bullet Seed is Treecko's STAB move of choice, as LC's damage roll mechanics make it the most powerful option as long as it hits at least three times. It also has use in that it can break past Sturdy, Focus Sash, and Substitute as well as defeat various Pokemon with the Weak Armor ability, such as Onix and Dwebble, though Treecko should be wary of hitting Vullaby with the move. Drain Punch gives Treecko a reliable way to break past the Steel-types that would otherwise wall it, 2HKOing Ferroseed and OHKOing Magnemite and Pawniard after a boost. Acrobatics becomes quite powerful once Treecko has boosted its Attack and consumed its Berry Juice and is useful for KOing Fighting-types such as Timburr, Croagunk, and Mienfoo. It also hits Bug-types such as Larvesta and Surskit and opposing Grass-types like Snivy and Pumpkaboo super effectively. Rock Slide is an option over Acrobatics, as it allows Treecko to OHKO Ponyta and Vullaby, which otherwise wall it, after a boost and Stealth Rock damage. It also hits Bug- and other Flying-types super effectively, though it does leave Treecko quite vulnerable to Croagunk and other Fighting-types.
Set Details
Berry Juice alongside Unburden secures Treecko's niche, as upon restoring its health with Berry Juice, Unburden will double Treecko's Speed and turn it into a threatening late-game sweeper. 116 EVs invested in HP alongside 156 Defense EVs allow Treecko to avoid OHKOs from some foes' super effective coverage attacks, such as Timburr's Ice Punch and Chinchou's Ice Beam, and trigger the consumption of Berry Juice. It can also help Treecko take some priority attacks after having set up. 36 EVs are invested in Speed to allow Treecko to hit 14 Speed. This allows it to outpace the entire unboosted metagame after Unburden activates, as well as some of the tier's fastest Choice Scarf users, such as Gastly and Doduo. 156 of the remaining EVs are invested in Attack with an Adamant nature to give Treecko as much of an offensive presence as possible and to allow it to break through the majority of the metagame after a Swords Dance boost.
Usage Tips
Treecko functions best as a late-game cleaner after its checks, like Ponyta, Vullaby, and Alolan Grimer, have been either removed or weakened. Before said checks are taken care of, it is important that Treecko doesn't take any chip damage so as not to activate Unburden too early. It should also refrain from getting hit with Knock Off in order to prevent the early activation of Unburden. Setting up is a rather tricky task for Treecko, as it should often try doing so against a foe whose coverage can whittle it into range of Berry Juice consumption but won't OHKO it. Chinchou, Drilbur, and Surskit all carry super effective coverage that won't flat-out OHKO Treecko, which makes these threats decent setup targets. U-turn users like Mienfoo and Vullaby can also bring Treecko into range of Berry Juice consumption, though some, such as Larvesta and Rufflet, can OHKO it with the move. Another setup strategy that is much less risky is using Swords Dance against foes with generally weak attacks, like Spritzee and Onix. Treecko will often be relying on Memento support, meaning that some Pokemon that carry super effective STAB attacks can also make decent setup targets with lowered offenses. Examples include Foongus, Croagunk, and Larvesta.
Team Options
Although it is indeed threatening once boosted, Treecko does require a lot of support to successfully sweep. Diglett is easily Treecko's best possible partner, as it is capable of supporting it in many different ways. It removes both Poison-types and Ponyta and most importantly can support Treecko with Memento, thus giving it a reliable setup opportunity. If Diglett isn't wanted or needed, Memento support is still highly recommended for Treecko, as it gives it setup opportunities against the likes of Foongus and Croagunk. Alolan Grimer and Cottonee can also provide Treecko with Memento support, while Pancham can also work as a Parting Shot pivot. Fire-type checks are highly recommended as teammates, as they can easily take care of Ponyta to give Treecko an easier time sweeping. Staryu, Mudbray, and Chinchou all work as reliable Pokemon for this role. Poison-type checks are also a necessity, as Treecko is unable to dent most Poison-types. Drilbur, Mudbray, and Abra can all function here. Answers to Flying-types, such as Onix, Elekid, and Magnemite, all make useful partners, as they can dispose of the likes of Archen and Doduo to help Treecko set up without fear of switch-ins to the aforementioned threats. Knock Off support is quite helpful as well, as the removal of bulkier foes' Eviolites gives Treecko an easier time cleaning up late-game. Alolan Grimer is a particularly good user of Knock Off in terms of supporting Treecko, as it can also dent one of Treecko's biggest annoyances in Spritzee with Poison Jab. Other examples of Pokemon capable of providing Knock Off support include Mienfoo, Vullaby, and Croagunk. Entry hazard support from the likes of Onix, Dwebble, and Omanyte allows Treecko to OHKO Ponyta with a boosted Rock Slide, as well as have an easier time cleaning up.