Dewgong

Type Tier
Water / Ice UU
Statistics
Stat
HP
90
383
Atk
70
238
Def
80
258
SpA
70
238
SpD
95
288
Spe
70
238
Name Item

Basic Dewgong

Leftovers
Moveset
~ Surf
~ Ice Beam
~ Rest
~ Encore / Sleep Talk

This set is pretty straightforward: Surf and Ice Beam for STAB, Rest to heal, and Encore is handy to take on Cursers. Sleep Talk is a decent option, since it has pretty good defenses.

Other Options

Perish Song and Whirlpool go hand in hand, you just need to hope Whirlpool hits and lasts the turns it needs for Perish Trapping to work. Toxic isn't really that good, but you might be able to work it in.

Opinion

Dewgong is a nice Pokémon to have. It is not recommend it as a Ground counter, however, since it can't take many hits on the switch in. Take caution when using it, and remember, it isn't Starmie or Lapras.

Counters

Electric Pokémon easily kill it. Snorlax can set up a Curse sweep against Dewgong, but has to worry about Encore giving a free switch. Blissey can counter it if it has Thunderbolt. Fighting-types such as Machamp or Heracross, or some other physical Pokémon not weak to Surf / Ice Beam that can hit hard will not have much trouble with Dewgong.