Hide Illumise

[HIDE="Skeleton"][Overview]

Good support Pokemon.
Prankster gives it its main niche.
Almost always sets up weather.
Mediocre bulk at best.
Works well in creating free turns and switch-in opportunities for teammates .with Encore and U-turn.
Has Baton Pass, but has very little worth passing.
Basically a less versatile Volbeat.
[SET]
name: Weather Supporter
move 1: Rain Dance / Sunny Day
move 2: U-turn
move 3: Encore
move 4: Thunder Wave
item: Damp Rock / Heat Rock
ability: Prankster
nature: Bold / Relaxed
evs: 248 HP / 252 Def / 8 SpD

[SET COMMENTS]

Rain Dance and Sunny Day are main moves to support the team.
Slow U-turn lets Illumise take a hit and get a teammate in safely in as few turns as possible.
Encore creates free set up and/or switch-in opportunities.
Thunder Wave cripples Pokemon that you don't want to Encore, mainly Choiced Pokemon.
Encore and Thunder Wave are generally best used before setting up weather to prevent wasted weather turns.
Prankster is key to success of set.
[ADDITIONAL COMMENTS]

Damp/Heat Rock are best choice of item to extend duration of respective weather.
EVs maximize physical bulk since Bug has decent physical resistances in Fighting, Ground, and Grass (mostly former two).
Roost is a viable option over Thunder Wave to extend Illumise's longevity.
Can set Speed IVs to 11 to still outpace no Speed Alomomola with a Relaxed nature, but makes U-turn go second more often.
Weather sweepers are a must: Ludicolo, Seismitoad, and Gorebyss are good Swift Swim users for Rain, while Exeggutor, Victreebel, and Shiftry are great Chlorophyll users for Sun.
Other bulkier weather setters are recommended for continued weather setting: Rotom-S for Rain and Regirock for Sun are good options.
Rain teams can abuse Pokemon like Jynx and Articuno due to reduced Fire weakness and Rain's additional benefits (Dry Skin and Hurricane).
Sun teams can abuse Solar Power Charizard as well as other Fire-types.
Something to absorb Toxic Spikes for Rain teams is helpful since they greatly reduce Rain teams lifespan by quickly wearing down sweepers.
[Other Options]

Moonlight provides recovery, but Illumise still takes over 50% damage from most powerful neutral attacks.
A Baton Pass set is viable, but all it really can Baton Pass are Charge Beam and Substitute.
A Swagger and Thunder Wave set is viable, but Liepard is far better thanks to Foul Play.
Toxic has priority with Prankster, but weather teams force many switches that decrease Toxic's potency.
Psych Up can copy opposing stat boosts to Baton Pass, but is very situational.
Fake Tears and Charm have priority and can weaken the opponent before they attack.
Tailwind can be used, but Thunder Wave is generally more useful.
[Checks and Counters]

Murkrow is Illumise's biggest nemesis thanks to Prankster Taunt and Brave Bird, as well as higher Speed.
Rock Blast users like Golem and Gigalith can limit the amount of set up Illumise can perform, as well as break its Substitutes.
Encore users without priority can go second and lock Illumise into a move.
Preying on its weaknesses is a simple way to reduce Illumise's set up to a single time.
Roar, Whirlwind, and Haze users can remove any boosts Illumise has set up.
Taunt users can limit what Illumise can do.
Liepard can use Prankster Taunt as well, or at least Swagger it and make it a matter of luck whether Illumise can set up or not.
It can also set up a different weather and ruin Illumise's work.
Stealth Rock reduces the number of switch-ins that Illumise gets.
[/HIDE]