Huntail
Swift Swim
Speed doubles in rain.
Type Tier
Water UU
Statistics
Min- Min Max Max+
HP
55
 - 251 314 -
Atk
104
 219 244 307 337
Def
105
 221 246 309 339
SpA
94
 201 224 287 315
SpD
75
 167 186 249 273
Spe
52
 126 140 203 223
Name Item Nature

Choice Band

 Choice Band Adamant
Moveset EVs
~ Aqua Tail / Waterfall
~ Ice Fang
~ Crunch
~ Return
6 HP / 252 Atk / 252 Spe

He has a few options to use his decent physical attack now, so this is rather obvious. Return is somewhat of a filler, but Water/Normal is unresisted by anything Huntail is likely to face.

Name Item Nature

Rain Dance

 Damp Rock / Life Orb Adamant
Moveset EVs
~ Rain Dance
~ Waterfall
~ Ice Fang
~ Return / Crunch
6 HP / 252 Atk / 252 Spe

Boost his poor Speed with Swift Swim and attempt a sweep. Crunch isn't needed here as Waterfall does more damage to most of Psychics and Ghosts Huntail will face. However, if you're worried about Shedinja or think you can take Slowking, Crunch can still be useful.

Other Options

He can Baton Pass, but with only Substitute and Barrier to pass it is a lesser option.

EVs

Max out Speed and Attack, Adamant nature. Jolly is an option on the Choice Band set if you wish to beat the base 60 group, who sit at 219 without a boosting nature.

Opinion

The Choice Band set hits fairly hard in the lower tiers, and he should take at least one decent physical hit thanks to his good Defense, but terrible HP means you can't afford to keep trying this. Huntail is just a very forgettable Pokémon.

Counters

Huntail can't compete against the bigger threats in the game, where his list of counters would be huge. Most Electric-types can beat the Swift Swim set, although Waterfall can leave a big dent in their HP. Lanturn is the best option thanks to his Water resist. Pelipper can bring him down with Shock Wave/Hidden Power Electric and takes hits from Huntail decently.