Ariados

Insomnia
Prevents sleep.
Swarm
Boosts power of Bug moves 50% when at 1/3 HP or less.
Type Tier
Bug / Poison NU
Statistics
Min- Min Max Max+
HP
70
- 281 344 -
Atk
90
194 216 279 306
Def
70
158 176 239 262
SpA
60
140 156 219 240
SpD
60
140 156 219 240
Spe
40
104 116 179 196
Name Item Ability Nature

Baton Passer

Leftovers Insomnia Impish
Moveset EVs
~ Sludge Bomb
~ Signal Beam
~ Spider Web / Agility
~ Baton Pass
252 HP / 42 Atk / 108 Def / 108 SpD

Here we have a Pokémon with the same stats as Mightyena, only slower. Not good, but Ariados can be a good starter and physical sweeper if you use it correctly. Double STAB is always good, particularly Signal Beam (Bug attacks come in short supply, you see), and with that decent Attack, Ariados can do some good damage if it hits the right Pokémon. A 10% chance of confusion is always handy as well.

Right, start off the battle with either Spider Web or Agility (I would go with Agility), and start attacking. When the time has come to save this spiders ass, you can baton pass the speed or trap onto another Pokémon to benefit from. Simple, and quite effective, just watch out for its weaknesses, like his old buddy Golem.

Ariados also works well enough primarily as a baton passer. Try using both Agility and Spider Web, attack with Signal Beam, and pass on the goodies. And always remember that Ariados does not fear sleep moves, so predict your moves right to cancel out that stuff. And need I mention that Ariados is a fantastic counter to Breloom?

Other Options

Substitute is something else that can be passed (Double Team can also be passed if evasion moves are allowed). Giga Drain can be used for Golem, Sandslash, and the other Pokémon that defend its physical assault. Ariados can also Toxic and trap the opponent, although it's a strat that rarely works for me. Thief and Hyper Beam are your standard suicide moves, really, both work well enough on this Pokémon.

EVs

Well, if you're going with the physical sweeper set, you will want to boost that attack a lot. But put a fair bit into that HP as well, so it can actually baton pass something as well. If you are using it primarily to baton pass, it's defences should be your area of focus.

Opinion

Another one of those bad Bug Pokémon you find at the start of the game, but it can actually work well in a UU battle. That decent attack power, coupled with two great attacks is sure to screw a few Pokémon over, it can baton pass a few nice things to other Pokémon to benefit from, and it completely stops Hypno, Parasect, Jumpluff and friends from mutilating you (temporarily.) Yes, it is a weak Pokémon, but use it right and it can make quite a positive difference to your team.

Counters

Both defenses are horrible, and none of it's attacks (even Giga Drain) will do serious damage to Golem, Sandslash, and other hard skinned Pokémon resistant to both STAB attacks (who usually hold Rock Slide anyway.) Also be wary of fast sweepers like Fearow, Mr. Mime and friends... whether you have Spider Webbed it or not, you will need to get Ariados out of there. Also Skarmory.