Gardevoir is a meh scarfer. The sole reason to use Gardevoir in the first place is for its Mega form and your Mega slot is taken up by Medicham. Have you considered Alakazam? It's frail, sure, but it still has the Speed to revenge kill threats:
-Psyshock
-Shadow Ball
-Focus Blast
-Hidden Power (Ice)/Dazzling Gleam/Thunder Wave
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Focus Sash
Ability: Magic Guard
Medicham can't afford the time to set up Bulk Up because of its frailty. This slot is better devoted to Substitute (to ease prediction on a forced switch or to avoid status) or Fake Out (to Mega Evolve safely).
A more efficient spread for Wobbuffet is 28 HP/228 Def/252 SDef. This helps its physical bulk, as well as its special bulk.
Psyshock > Psychic on Latios; dealing with Blissey/Chansey helps big-time.
The pick between Slowbro or Slowking depends on who you have a harder time with. If you struggle against physical threats, use Slowbro; the same can be said for special threats with Slowking:
Slowbro:
-Scald
-Psychic/Thunder Wave
-Ice Beam
-Slack Off
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers
Ability: Regenerator
Slowking:
-Scald
-Psychic/Dragon Tail
-Fire Blast
-Power Gem/Ice Beam
Nature: Calm/Sassy
EVs: 248 HP/164 SAtk/96 Spe
Item: Assault Vest
Ability: Regenerator
Metagross is terrible in OU now; Aegislash does what it can do better. Since this is a mono-Psychic team, use Espeon, as it has Magic Bounce to discourage status moves:
-Reflect
-Light Screen
-Psychic/Psyshock/Yawn
-Baton Pass
Nature: Timid
EVs: 252 HP/4 Def/252 SDef
Item: Light Clay
Ability: Magic Bounce