Dustox

Shield Dust
Secondary effects won't occur.
Type Tier
Bug / Poison UU
Statistics
Min- Min Max Max+
HP
60
- 261 324 -
Atk
50
122 136 199 218
Def
70
158 176 239 262
SpA
50
122 136 199 218
SpD
90
194 216 279 306
Spe
65
149 166 229 251
Name Item Nature

Some set

Leftovers / Choice Specs Calm / Modest
Moveset EVs
~ Bug Buzz
~ Toxic / Sludge Bomb
~ Roost / Psychic
~ Light Screen / Hidden Power Fire
252 HP / 252 SpA / 6 SpD

Of all the sets you can use on Dustox, this is probably one of the better ones. It can switch in on special attacks, use decently powered STAB moves (off 199 SpA), poison, support, and heal. Hidden Power Fire will hurt Mawile. Dustox can use Choice Specs if you want.

Other Options

U-turn can scout incoming Pokémon and can prove to be a useful STAB move.

EVs

Just focus on Dustox's HP and Special Attack. Speed investment is futile because it's slow anyways.

Opinion

Dustox really is not something you want to use in competitive play. Even in NU there are better Pokémon to use. Choice Specs allows Dustox to actually have decent Special Attack this generation and can do decent damage to many NU Pokémon.

Counters

Dustox is slow (though maybe not THAT slow in NU terms), has a couple common weaknesses (including Stealth Rock), and has lame Attack. Any Pokémon that can take a STAB Sludge Bomb or Bug Buzz is a Dustox counter.