- If allied with a Pokemon with Plus or Minus, the wielder's Special Attack increases by 50%.
Level 100 Statistics (see level 5, 50, 100)
Minun isn't "awful," but it doesn't really offer anything over Raichu, Pikachu, or even Plusle, either.
What Minun does offer over its Electric mouse brethren is the ability to actually take a hit and pass a Nasty Plot boost to a teammate. Get Minun into battle, preferably on a setup move like Stealth Rock, Encore it, set up Nasty Plot as the opponent switches, and high tail it out of there with Baton Pass. An uninvested +2 Thunderbolt can still actually hurt things, and Minun is decently fast, so don't count it out of the battle once it's fulfilled its duty.
A good teammate is one that is bulky enough to weather an opponent's incoming hit and hopefully be able to outspeed the opponent once it's in to sweep. Minun tends to lure Ground-type attacks and physical attacks in general, making Pokemon like Articuno, Charizard, and Rotom-S natural teammates. Tangela and Exeggutor are also decent choices, though they are pretty slow and will likely have to withstand two attacks before being able to attack.
An offensive Nasty Plot set is doable, but Raichu and Plusle do it better. Minun gets Wish, but it has a pretty meek HP stat. A general annoyer set of Substitute, Thunder Wave, Thunderbolt, and Fake Tears can give the opponent some trouble if you can get a bunch of entry hazards on their side of the field.
Checks and Counters
If you inadvertently gave Minun the ability to switch in freely on a status move, save yourself the trouble of getting Encored and switch out. You need to do everything in your power to prevent that Baton Pass from happening (remember that Pursuit does not work on Baton Pass, so don't have that as your solution). Failing that, you need to smite whatever Minun is going to Baton Pass to. Minun in and of itself isn't very threatening unless it's packing heat, i.e. a Life Orb (so don't switch in a Water-type into Minun regardless), but there are quite a few Pokemon in NU that can be pretty scary with +2 Special Attack.