Battle Moveset

A Good Moveset for Gardevoir would have to be this:

Attacks:

Psychic
Calm Mind
Thunderbolt / Ice Punch / Fire Punch / Hidden Power [Water]
Will-o-Wisp / Thunder Wave / Hypnosis / Rest

Items Attached:

Leftovers

Preferred Trait:

Trace

Preferred Nature:

Modest

Strategy Using Gardevoir

The joint sweeper-special wall Gardevoir set, probably the most generally seen set up. After a single Calm Mind Gardevoir makes for a suitable special wall while with Will-o-Wisp it's capable of taking special hits (except for ones with the Guts trait).With Thunder Wave it can cripple all sweepers while with Hypnosis it can leave a Pokémon unable to retaliate, although its accuracy leaves a lot to be desired.

Gardevoir has a nice variety of special attacks to backup Psychic, Thunderbolt being its most powerful but with the others having their situations of use: Ice Punch for Salamences, Celebis and Ground types, Fire Punch for Metagrosses and Hidden Power [Water] for Houndooms and Tyranitars.

Trace is a trait unique to Gardevoir and Porygon2 and with properly timed prediction can be used to recover HP (taking advantage of Volt and Water Absorb traits from the likes of Jolteon and Vaporeon), as well as being used to Trace Natural Cures, giving potential for Rest to be used.

EV Corner:

EVs: 172 HP / 136 Def / 20 Spd / 46 SAtk / 136 SDef
Modest Nature (+SAtk, -Atk)

320 HP, 200 Def and 300 SDef make Gardevoir capable of taking a fair few hits. 20 Speed allows it to outrun standard Metagross among other things while Modest and a few SAtk EVs give Gardevoir a solid Special Attack to launch special attacks off prior to Calm Mind.

Other Optional Sets

Gardevoir @ Leftovers
Trait: Trace
EVs: 236 HP / 20 Spd / 118 SAtk / 136 SDef
Modest Nature (+SAtk, -Atk)
- Psychic
- Wish
- Thunder Wave / Will-o-Wisp
- Hypnosis

More of a support Gardevoir, using it's access to all negative statuses to its advantage, working on the "double-spore" method, putting something to sleep and then paralysing or burning the next Pokémon to be switched in. Wish aids both Gardevoir's tanking ability as well as the rest of the team.

Strategy Against Gardevoir

Gardevoir's weak point is in its physical defence, a weakness that can be overcome with the use of Will-o-Wisp (or even the lesser seen Reflect and Charm). Heracross is one of the few Pokémon who can take advantage of being burned, negating the halving of its attack stat while maintaining a 50% boost in offensive stats while also outrunning Gardevoir and having Megahorn for an OHKO. Unfortunately, it risks taking a super effective Psychic as it comes into play and makes for a much riskier 'counter'. Celebi and Blissey can force it into a stall war, Psych Up Regice stands a fair chance of coming out on top although will lose without the aid of Psych Up while Snorlax can beat it hands down with physical offence. Dragon Dancing Tyranitar will beat non-Hidden Power [Water or Grass] Gardevoir provided it's fast enough to Taunt before a Will-o-Wisp or Thunder Wave. Faster Metagrosses (although a bit more uncommon) will force a switch, Houndoom will cause lots of problems if it can switch into Gardevoir prior to it Calm Minding. The majority of physical sweepers that won't take a huge hit from Gardevoir's offences are more than capable Gardevoir counters.

Contest Moveset

A good Contest Moveset for Gardevoir would have to be this for the Smart Contest best with Calm, Gentle, Careful, Sassy Nature:

Attacks:

Calm mind
Psychic
Destiny Bond
Thunderbolt

Items Attached:

Green Scarf

Strategy Using Gardevoir

If you do the attacks in this following order you should have very few problems; 1st - Calm mind, 2nd - Psychic, 3rd - Calm mind, 4th - Thunderbolt, 5th - Destiny Bond

Locations in Games

Ruby/Sapphire/Emerald Evolve Kirlia Colosseum/XD Trade from RSEXD (Col), Evolve Kirlia (XD) Fire Red/Leaf Green Trade from RSEXD

Animé Appearences

Gardevoir has had an Animé Appearence. In it, it was the parent of a Ralts that is rescued by Max Episode 386: Rescue Ralts! Hurry Max!