Dewgong
Thick Fat
Halves Ice and Fire damage.
Type Tier
Water / Ice NU
Statistics
Min- Min Max Max+
HP
90
 - 321 384 -
Atk
70
 158 176 239 262
Def
80
 176 196 259 284
SpA
70
 158 176 239 262
SpD
95
 203 226 289 317
Spe
70
 158 176 239 262
Name Item Nature

Diverse Water

 Leftovers Calm
Moveset EVs
~ Ice Beam
~ Surf
~ Encore / Sleep Talk / Toxic
~ Rest
252 HP / 144 Def / 96 SpA / 16 SpD

Dewgong is similar to other bulky Waters. Surf and Ice Beam both get STAB, which is always great. Even with a moderate Special Attack stat, it will do high amounts of damage to many other NU Pokémon. Rest is standard on Dewgong to give it the ability to regain HP. Encore can screw up many things, forcing Pokémon that used Substitute or tried to stat up to switch out. Toxic may be a common move, but it's still great to help Dewgong stall and do more damage. If you are unsure, Sleep Talk is always a nice alternative, giving Dewgong the ability to attack when it is Sleeping.

Other Options

Signal Beam and Body Slam would require you to have a -Speed nature instead of -Attack. Signal Beam won't come in handy much, but can catch a random Crawdaunt off-guard. Body Slam is for Paralysis and should not be used in conjunction with Toxic. Perish Song is good for when all your opponent has is a last Pokémon, or to force something out. Swagger is dangerous to use but can pay off at times, and will commonly force switches.

EVs

Dewgong can pound many common NU Pokémon with STAB attacks, so not much Special Attack is needed; 200 is all you need. Max out HP, as that is going to help him take attacks. Using the rest mainly in Defense and the leftover EVs in Special Defense is also beneficial for defensive play.

Opinion

Dewgong is often overlooked, but does the job of NU bulky Water quite well with defensive stats far exceeding most other NU Pokémon. Two powerful STAB attacks and Toxic make Dewgong more of a threat.

Counters

In NU: Kecleon will beat you easily. Pelipper often carries a super effective Hidden Power Electric or Grass. Other Resting Waters such as Wailord will counter Dewgong easily. Swalot will stall you as well.

In UU: Water types with Rest will stall Dewgong. Tentacruel is immune to Toxic and takes very little damage from Surf and Ice Beam and can set up on Dewgong; Encoring its Substitute or Swords Dance will however force a switch.

In OU: A Milotic with Rest will stall Dewgong without a problem, Curse Swampert (preferably with Rest) will scare away Dewgong. A bulky Zapdos can even come in on super-effective Ice Beams easily and kill with Thunderbolt. Raikou is not hit super effectively and has high enough Special Defense and HP so that it can come in on Dewgong's Surfs or Ice Beams... just watch out for Toxic.