Moves
Return and Seed Bomb serve as Deerling's two STAB moves and capitalize on its unique type combination. Both hit quite hard thanks to Life Orb, especially after a Sap Sipper boost. Jump Kick hits Steel-types, which resist both of Deerling's STAB moves. In the last slot, Substitute lets Deerling take advantage of the switches it forces, as well as check Ferroseed with no fear of getting paralyzed. Wild Charge lets Deerling lure in and remove Archen, which would otherwise wall it. Baton Pass gives Deerling a way to keep momentum alive by pivoting out on one of the switches it forces to bring in the appropriate counter and can let Deerling pass the Attack boosts it gets from Sap Sipper off to a teammate. Baton Pass can also be run over Jump Kick on teams that don't really require coverage to hit Steel-types.
Set Details
Sap Sipper gives Deerling a pseudo-immunity to Grass-type moves, allowing it to check Snivy, Cottonee, Ferroseed, and other common Grass-types. The given Attack EVs, plus Life Orb, allow Deerling to make the most use of the Attack boosts it gets from Sap Sipper, as well as force out the Pokemon it checks and hit the switch-ins as hard as possible. Maximum Speed EVs and a Jolly nature are used so Deerling can hit 18 Speed, outspeeding a majority of the metagame, most notably Snivy. The remaining EVs go into HP and Defense to bolster Deerling's physical bulk.
Usage Tips
This set works best by capitalizing on Deerling's unique ability to wall certain Pokemon: Ferroseed, Cottonee, and Snivy, specifically. Deerling should be sent in against these Pokemon to get a Sap Sipper boost if possible and then force the Pokemon out. Deerling can also be used to force out Chinchou and, to a lesser extent, Magnemite. However, Deerling does need to be wary of status moves, such as Snivy's Glare and Chinchou's Scald. Additionally, due to its excellent Speed tier, Deerling can be used to revenge kill weakened opposing Pokemon throughout the match. This Deerling set struggles against opposing priority, and so it should usually be switched out of Pokemon such as Pawniard, Fletchling, and Timburr, which carry powerful priority moves.
Team Options
Because of the switches that Deerling forces, teammates that set up entry hazards make for good partners. Onix and Drilbur are good options that can set Stealth Rock for Deerling, and in return they appreciate having Water- and Grass-type attacks sponged that they're weak to. Onix can also check the Fire- and Flying-types that make Deerling's life miserable. Fighting-type checks such as Skrelp and Snubbull are necessary for Deerling to do its job. Skrelp and Deerling form a very solid resistance core as well. Likewise, a Fletchling check is absolutely essential. Archen, Omanyte, and the aforementioned Onix counter Fletchling while retaining good synergy with Deerling. Pokemon that appreciate a +1 Speed boost Baton Passed to them such as Choice Scarf Drilbur and Shellder all make for good partners as well. Lastly, sweepers that need Grass-types worn down, such as Corphish, appreciate Deerling doing so.