Cacnea is one of the most threatening Swords Dance sweepers in LC since it has access to Sucker Punch, the strongest priority move in the game. After one Swords Dance, Cacnea's already great Attack stat is boosted to a sky-high 38, allowing it to fire off deadly attacks that can OHKO many LC Pokemon.
Sucker Punch should never to be underestimated even without the STAB boost; Cacnea has two other options to choose from for the third slot: Focus Punch and Seed Bomb. The choice of secondary attacking move is dependent on your final move. If you select Substitute then Focus Punch is the preferred move as it is more powerful and provides better coverage than Seed Bomb, and it gives Cacnea a shield to ensure Focus Punch hits. It's also very helpful should your opponent try to out-stall Cacnea's Sucker Punch PP, as it gives you the perfect opportunity to set up Substitute. On the other hand, if you select Encore, then Seed Bomb should be used. This is because Encore doesn't provide a shield like Substitute does, so Focus Punch tends to be a riskier option. Swords Dance, Encore, and Sucker Punch are also a ridiculously good trio. If your opponent even thinks about stalling out Sucker Punch's PP, they'll be hit with Encore, easily giving you a free Swords Dance boost as they are forced to switch out. If they attempt to avoid Encore by attacking, they will be hit by Sucker Punch. Either way, Cacnea is always going to be in a win-win situation with either Substitute or Encore, so it's really up to personal preference.
The items listed are also going to be determined by your chosen moveset. If you opt to use Substitute, Oran Berry is the primary item as it lets Cacnea potentially set up without being at 1 HP afterwards or gives it more chances to fish for a miss due to Sand Veil (while a sandstorm is in effect), which gives it the chance to set up a Substitute. Focus Sash is suggested if you are using Encore, as Substitute renders your Focus Sash useless. If you do decide to use Focus Sash then it also guarantees Cacnea at least one Swords Dance boost, as long as entry hazards are not present. Even though Oran Berry and Focus Sash are the preferred items, Life Orb can also be useful. With Life Orb, Cacnea's attacks are powered up by 30%, making it a much larger offensive threat. This gives Sucker Punch that extra kick to OHKO Pokemon such as Porygon, Paras, and Wailmer, assuming Cacnea has used Swords Dance and Stealth Rock is on the field.
Since Cacnea has the ability Sand Veil, Hippopotas' sandstorm can be used to take advantage of the evasion boost so it can Swords Dance or Substitute easier. When it comes to sweeping, Duskull can stop Cacnea cold if it lacks Seed Bomb. Duskull's amazing Defense, immunity to Focus Punch, and access to Will-O-Wisp will scare off Cacnea with ease. Sucker Punch will be useless against it, as Duskull generally won't bother to attack Cacnea. To deal with Duskull, Houndour is by far the most effective counter, absorbing Will-O-Wisp for added fire power and destroying it with Dark Pulse or Fire Blast. Speaking of Houndour, if Cacnea lacks Focus Punch, Houndour can easily switch into Sucker Punch, Seed Bomb, or Swords Dance and eliminate it. Munchlax is a great switch-in to Houndour, thanks to its Fire-type resistance due to Thick Fat, great overall bulk, and access to super effective Earthquake. Croagunk will always be a troublesome threat no matter which attack Cacnea decides to use, as it is resistant to all of them. It's best to pack a Pokemon who can dispose of Croagunk, or Cacnea will have a hard time sweeping. Diglett and Gligar are both excellent options, the former trapping it because of Arena Trap and finishing it off with Earthquake, while the latter switching into almost anything Croagunk uses (excluding Ice Punch) and destroying it with Earthquake.