Steelix is really easy to get. You just need to catch a Luvdisc and you can trade for one in Cyllage City, level 20. Steelix might not be a good option though, considering that her Gardevoir knows Shadow Ball, and Steelix has bad sp. def along with no resistance to Shadow Ball. However, you might want to bank on Gyro Ball dealing a load of damage since Steelix is so slow.
Lucario is fine since it has Metal Claw, Shadow Claw & Shadow Ball, but it has no resistances to any of Gardevoir's moves (T-Bolt, Shadow Ball, Moonblast, Psychic). Metal Claw is at base 50 power as well, though I guess you could just mega evolve for that base 145 attack. Lucario also has only base 70 sp. def, which is better than Steelix's, but still poor when up against Mega-Gardevoir's base 165 sp. atk.
Lucario @ Lucarionite
Ability: (doesn't matter)
-Metal Claw/ Calm Mind
-Close Combat/ Aura Sphere
-Shadow Claw/ Shadow Ball
That's the set I recommend for dealing with Diantha. Lucario can learn Calm Mind as well, so it might be worth while to pair that with Shadow Ball. Lucario will also out speed Gardevoir, as will its mega form out speed Gardevoir's mega form.
Alternatively, you can use these two Fairy types to counter her Gardevoir easily. It only has special moves, so set up a Light Screen, spam some Calm Minds and you're good to go.
Klefki @ Light Clay/ Metal Coat
-Mirror Shot (Flash Cannon is post game)
-Light Screen/ Dazzling Gleam
Sylveon @ Leftovers/ Spell Tag
Ability: Cute Charm
-Light Screen/ Clam Mind
Those are my suggestions anyway.