Moves
Earthquake is Trapinch's primary STAB move, heavily damaging most Pokemon that do not resist it. Feint guarantees that Trapinch can pick off weakened threats such Focus Sash Abra and Sturdy Magnemite; it is used over Quick Attack because the difference in damage is negligible for its purpose with the given EV spread, and Feint can let Trapinch get some chip damage in on opposing priority users. Rock Slide provides coverage against Flying- and Bug-types, although it lacks the Base Power to significantly damage bulkier Pokemon. The last moveslot is just a filler option; Toxic ensures that Calm Mind Spritzee does not set up on Trapinch, whereas Superpower threatens Porygon.
Set Details
The given defensive EVs ensure that Life Orb Adamant Pawniard's Knock Off and Analytic Magnemite's Flash Cannon almost never OHKO Trapinch after Stealth Rock. 18 Attack lets Trapinch 2HKO Eviolite-less Spritzee the vast majority of the time. Berry Juice is a strong item on Trapinch because it usually lets Trapinch heal back to full HP after trapping its target; it also allows Trapinch to directly switch into Choice Scarf Magnemite and reliably revenge kill SturdyJuice Magnemite. Eviolite is another good choice to better deal with Knock Off users and avoid a greater variety of OHKOs, in exchange for Berry Juice's longevity. If Eviolite is run, an EV spread of 156 HP / 36 Atk / 76 Def / 236 SpD is preferable to usually avoid the 2HKO from Magnemite's Flash Cannon after Stealth Rock before factoring in Analytic, and the OHKO from Choice Scarf Chinchou's Hydro Pump after a layer of Spikes; the physical bulk to withstand Pawniard is no longer needed, as Eviolite reduces Knock Off's damage. A more offensive EV spread of 156 HP / 196 Atk / 76 Def / 76 SpD with either Berry Juice or Eviolite is viable to 2HKO Ponyta while burned and KO offensive Porygon after Stealth Rock with a combination of Superpower and Quick Attack, but the lower bulk makes it more difficult for Trapinch to trap its other targets; this is one of the few scenarios where Quick Attack's extra power makes it worth using over Feint.
Usage Tips
If Trapinch is required to trap more than one Pokemon on the opposing team, it is best to use it as a revenge killer, as Berry Juice will usually allow it to heal back to full health after trapping one target. With Feint, Trapinch can ensure that heavily weakened targets that are affected by Arena Trap are picked off. Do not directly switch Trapinch into Magnemite until you can be sure it is a Choice Scarf variant, has Sturdy broken, or has used up its Berry Juice, as if a healthy SturdyJuice Magnemite hits Trapinch on the switch with Flash Cannon, it will be able to win; however, Trapinch can reliably revenge kill all variants of Magnemite. Ponyta needs to be hit with Knock Off before Trapinch can trap it, and Trapinch will be heavily weakened in the process. Be careful of the opponent taking advantage of Trapinch's predictability and using double switches to gain momentum on predicted Trapinch switch-ins.
Team Options
Trapinch's niche is its ability to remove key Pokemon such as Focus Sash Abra, Magnemite, Diglett, Gothita, Pawniard, and Croagunk; because of this, Pokemon that appreciate these threats gone like Croagunk, Fletchling, and Foongus enjoy having Trapinch as a teammate. As Trapinch tends to be easily forced out once it has trapped something, teammates such as Foongus and Chinchou that can deal with various Water-, Grass-, and Flying-types are necessary to ensure that it does not become a liability. Paralysis support from the likes of Snubbull and Foongus can make it easier for Trapinch to revenge kill frailer targets. Slow VoltTurn users such as bulky variants of Mienfoo and Chinchou can help to get Trapinch in safely. Entry hazard removal, especially a means to remove Toxic Spikes, can help Trapinch to more consistently avoid specific KOs.