Battle Moveset

A Good Moveset for Illumise would have to be this:

Attacks:

Thunder Wave
Wish
Seismic Toss
Charm / Light Screen / Counter

Items Attached:

Leftovers

Preferred Nature:

Calm

Strategy Using Illumise

Illumise is by all means an NU Pokémon and barely usable. It lacks effective STAB and has a somewhat limited move pool. Wish and Thunder Wave are the best moves at it's disposal to fill a 'supportive' role. Seismic Toss will be your most successful means of dealing damage, it lacks attack to use STAB moves and lacks special attack to use non-stab moves effectively. The final move is just a mix of preference. Charm can help it deal with physical sweepers to some extent, Light Screen aids it in walling special attackers and goes hand-in-hand with it's passable special defence stat and Counter can take something by surprise, although it isn't worth counting on it.

EV Corner:

EVs: 196 HP / 60 Def / 252 SDef
Calm Nature (+SDef, -Atk)

Special defensive set-up. Probably the best you'll get with Illumise. Speed is optional and may prove helpful to outrun tank Pokémon with higher base speeds or to outrun slower sweepers (16 Speed EVs will allow it to outrun a max speed Adamant Machamp for example).

Other Optional Sets

Illumise (F) @ Leftovers
Trait: Oblivious
EVs: 196 HP / 252 Spd / 60 SDef
Timid Nature (+Spd, -Atk)
- Encore
- Substitute
- Psych Up / Wish
- Thunderbolt / Seismic Toss

An incredibly situational set. Encore something that has used a stat-boost and is slower than you. A tank that has set-up multiple stat boosts but is slower (such as Calm Mind Slowbro) is a good example of what to take advantage of. Use Psych Up or Substitute and maintain Encore until they switch out while pulverising them with TBolt (it's most usable special attack) or Seismic Tosses. Most players will have wised up to your intentions on the use of Encore so the set carries added risk along with being situational. With weak defence there is little room for error as well so the set is very "do or die". You could lose the Psych up variation for Wish and just use Encore as a method to punish any slower Pokémon trying to use stat-ups.

Other Optional Moves

Quick Attack, Moonlight, Protect, Ice Punch, Giga Drain, Flatter, Baton Pass. Quick Attack is a simple "kill the End-Revver" move, other Pokémon can use it usefully. Moonlight is just Wish without the supportive aid and generally isn't worth it. Protect can work with Wish to guarantee you the heal in exchange for another move slot. Ice Punch and Giga Drain are just optional special attacks. Flatter has some use in 2v2 however in 1v1 it's just Confuse ray with the risk of aiding special based Pokémon. Baton Pass could work with a Sub-Berry or Psych Up set.

Strategy Against Illumise

It's not a particularly hard to counter Pokémon. It's special defence is passable but it lacks defence and HP. It's weak to Fire, Flying and Rock and resistant to Ground, Fight and Grass, although strong Ground and Fighting attacks will dent it regardless of resistance. It's offensive move set is limited so it's exposed to Taunt and Snatch. Most repetitive beatings will be adequate for handling it, be ready for potential Thunder Waves however.

Contest Moveset

A good Contest Moveset for Illumise would have to be this for the Cool Contest best with Lonely, Adament, Naughty or Brave Nature:

Attacks:

Rain Dance
Thunderbolt
Aerial Ace
Thunder Wave

Items Attached:

Red Scarf

Strategy Using Illumise

If you do the attacks in this following order you should have very few problems; 1st - Aerial Ace, 2nd - Rain dance, 3rd - Thunderbolt, 4th - Rain dance/Aerial Ace, 5th - Thunderbolt/Thunderwave

Locations in Games

Ruby/Sapphire/Emerald Route 117 Colosseum/XD Trade from RSE Fire Red/Leaf Green Trade from RSE

Animé Appearences

Illumise has had a few Anime Appearences. In it, a trainer owned an Illumise that another trainer's Volbeat was in love with: Episode 318: Love At First Flight