Hit like a sac of doorknobs (choice specs)
Item: take a wild guess (if you guessed choice specs, you were correct)
Ability: natural cure
Thunderbolt(thunder if on rain team)
Ice beam (blizzard if on hail team)
Equipped with choice specs, starmie's decent special attack is boosted to ridiculous levels, at the cost of being able to switch attacks. Timid is the preferred nature here, because it allows Starmie to outspeed more threats. However, if you want Starmie to hit as hard as possible and being outsped by a couple things does not matter to you, modest is also acceptable. Surf and hydro pump are Starmie's primary options. Both are viable, and whether you go for power or accuracy it will work. Unless starmie is on a rain team, thunderbolt goes in the next slot for dealing with water types. However, if starmie is given rain support, thunder, with its accuracy boosted in rain, is the best option. It should be noted that starmie loves the rain, as it allows it to run thunder and boosts its STAB moves. The third slot goes to ice beam for hitting grass and dragon types, completing the BoltBeam combo. However, Starmie is also great for a hail team, as it resist fighting, fire and steel, three of ice's weaknesses. If used in hail, blizzard is also an attack option. For the last slot, psyshock can be used as a secondary STAB for dealing with special walls, while trick allows starmie to be free of its choice item, making it the burden of the opponent. Other options include hidden power fire for ferrothorn, hidden power fighting for tyranitar, and hidden power grass for gastrodon. Hope this helped.