Pachirisu has pretty terrible offenses and no way to make them better outside of Charge Beam. It's defenses are better then average for an electric rodent an it should have an easier time staying alive. Generation V has given Pachirisu a much more useful ability in Volt Absorb. This gives Pachirisu a chance to switch in safely.

PachiFusion

Pachirisu @ Leftovers
Ability: Volt Absorb
EV: 252 Speed / 252 HP / 4 S. Defense
Nature: Timid (+ Speed / - Attack)

Super Fang
Thunder Wave
Swagger / Flatter
Volt Switch / U-Turn

Pachirisu can run a poor man's Parafusion with the help of Swagger or Flatter. Whichever 100% confusion move you use depends on your preference. Flatter sharply raises Special Attack, which makes it safer to use on suspected Physical Sweepers. Swagger boosts the Attack of a Pokemon and makes it more likely that they'll KO themselves, which is how Pachirisu will likely be netting KOs due to its pitiful attacking stats. Volt Switch / U-Turn are for when Pachirisu needs to exit the scene quickly. U-Turn provides a switch that cannot be commonly blocked while Volt Switch provides a stronger attack.

RestTalk

Pachirisu @ Leftovers
Ability: Volt Absorb
EV: 252 HP / 252 S. Defense / 4 Defense
Nature: Calm (+ S. Defense / - Attack)

Rest
Sleep Talk
Super Fang
Discharge / Toxic

This set is designed to make Pachirisu harder to kill then your average electric rodent. An Earthquake will still ruin Pachirishu's day, but it can take a hit or two from a Special Attacker or Wall. The idea is to weaken the opposing Team with a combination of Super Fang and your status of choice. If you had Toxic Spikes support you can even get away with using a Swagger in the last slot.

Hit and Run

Pachirisu @ Leftovers / Light Clay
Ability: Volt Absorb
EV: 252 Speed / 252 HP / 4 S. Defense
Nature: Timid (+ Speed / - Attack)

Light Screen
Super Fang
Volt Switch
U-Turn

Pachirisu does two jobs here. The most important is sinking its fangs into walls and other threatening Pokemon so your team mates can pick them off. Pachirisu can only set up Light Screen to further improve its passable Special Defense, so you can take advantage of that. Leftovers keep Pachirisu alive longer when constantly tagging in and out. If you don't value that surviability you can run Light Clay instead to prolong the effects of Light Screen.

Team Mates: Pachirisu can't KO much on its own. A Pokemon that can assist with the KO is essential. Toxic Spikes support can save it a moveslot and allow it to annoy another way. A Pokemon that can bait an electric attack can help Pachirisu get in. Pachirisu itself can be used to bait attacks, mainly Earthquake, which can help a Flying / Levitating team mate come in.

Counters: On most sets Taunt will prevent Pachirisu from doing anything. Prankster Pokemon will completely rain on Pachirisu's parade since most get Encore, Taunt, or both. Ground Types can stop a Volt Switch and either get a Free Turn or an Easy KO against Pachirisu's meager defense.