Ariados
Insomnia
Prevents sleep.
Swarm
Boosts power of Bug moves 50% when at 1/3 HP or less.
Type Tier
Bug / Poison UU
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 Pass

 Focus Sash / Leftovers Insomnia Adamant
Moveset EVs
~ Spider Web / Agility
~ Baton Pass
~ Poison Jab / Sucker Punch
~ Bug Bite / Sucker Punch
6 HP / 252 Atk / 252 Spe

Baton Passing Spider Web is Ariados' biggest "asset". However, he is incredibly frail and slow, so you may want to try Agility passing instead. Sucker Punch helps against faster Ghosts and Psychics, and Poison Jab and Bug Bite are there just because they're his best—and only—useful STAB options. Endure and a Salac Berry can help to get off a Spider Web pass.

Other Options

Shadow Sneak is viable if you think something will try and set up against Sucker Punch.

EVs

Max Attack and Speed.

Opinion

He has the following qualities: slow, fragile, poor typing, terrible movepool. "Because I want to" is the only reason you'd ever be likely to use this awful arachnid. If you can trap something and pass to a sweeper, then Ariados will have at least served some purpose to help the team.

Counters

Fire-types, Flying-types, Steel-types... countering Ariados really isn't that hard.