Mothim

Swarm
Boosts power of Bug moves 50% when at 1/3 HP or less.
Type Tier
Bug / Flying UU
Statistics
Min- Min Max Max+
HP
70
- 281 344 -
Atk
94
201 224 287 315
Def
50
122 136 199 218
SpA
94
201 224 287 315
SpD
50
122 136 199 218
Spe
66
151 168 231 254
Name Item Nature

Choice Specs/Scarf

Choice Specs / Choice Scarf Timid
Moveset EVs
~ Bug Buzz
~ Air Slash
~ Energy Ball / U-turn
~ Shadow Ball
4 Def / 252 SpA / 252 Spe

Fairly slow, but his movepool isn't too bad and his Special Attack is above average. Shadow Ball is mostly for Rotom who resists both STAB moves. Energy Ball hits Rock types coming in to absorb Air Slash. Bug Buzz hits most Rock types for at least neutral damage, so you can drop Energy Ball in favor of U-turn to scout your counter while getting in a light hit. Choice Specs is his only real hope of hurting something, but the Scarf might allow Mothim to get in a hit first and you might get some luck with the flinch rate of Air Slash.

Other Options

Psychic hits Poison types that resist his Bug STAB, but they take a good hit from Air Slash anyway. Hidden Power Fire or Ground helps to cover Steel types, while Hidden Power Rock helps against Flying Types. Roost can get back HP lost from Stealth Rock damage, but Mothim is better suited to all out attack.

EVs

4 Def / 252 SpA / 252 Spe is fine. Timid or Modest natures. You may want Timid even if you use Choice Scarf as Mothim's Speed is poor. Naive or Rash if you want U-turn to hit a little harder. The four extra points are not in HP as that would give Mothim an even HP. An odd HP is needed to make sure Mothim can survive two switch-ins into Stealth Rock.

Opinion

Bug/Flying types are notorious for being incredibly poor competitively and Mothim is no exception. Too many weaknesses to common moves and taking a massive 50% on each switch-in if Stealth Rock is in play really screws him. He's got a decent special movepool though and may get in a good hit or two before he dies. Essentially, Mothim is a weaker version of Yanmega.

Counters

Most Steels just laugh at him without the appropriate Hidden Power. Fire types can handle him well, although Air Slash does reasonable damage to some of the weaker ones. While Rotom fears Shadow Ball, he's otherwise a good switch-in on Mothim and destroys the bug with his STAB Thunderbolt. Just throw a super effective or decently powered neutral attack and Mothim should fall easily.