While I didn't use Gardevoir on my team - lack of space, I dropped Kirlia in the meanwhile, I have to agree: Kirlia is so hard to train, that slow exp.rate is really annoying.
I have used Zangoose, and it was a pleasant surprise.
Name/Tier: Zangoose -
B-Tier
Availability: Route 8 (Hordes), more common in X
Stats: 73 HP / 115 Atk / 60 Def / 60 SpA / 60 SpDef / 90 Spd in normal form
Zangoose is available fairly early, with a great attack and a good speed for in-game purposes. Its defenses are pretty low, though.
Typing: Normal. Pretty average. Ghost immunity is great, but the Fighting weakness is problematic.
Movepool: Having a STAB Return with that massive attack early-game is just amazing. It has cool coverage options like Shadow Claw, X-Scissor, Aerial Ace, Rock Smash (beggining)/Power-Up Punch (After the 3rd Gym)/Close Combat (Late-Game). Swords Dance works like magic with Zangoose. He learns great moves on the special side (BoltBeam) but with 60 Sp.Attack, just forget it.
Major Battles:
Grant: Rock Smash can help, especially vs Amaura.
Korrina: Stay away. Return can help, but there are much better options. Zangoose's Defenses aren't ideal.
Ramos: He's a joke anyway. Return all the way.
Clemont: Neutral Match-up. Magneton is a bit difficult to take down, but Power-Up Punch can help.
Valerie: Neutral. Her pokemon are frail on the physical side, so Return is your best bet. It won't do much to Mawile, though.
Olympia: Return/Shadow Claw/X-Scissor. Don't pick the latter against Sigilyph.
Wulfric: Close Combat. Avalugg has a massive Defense, so try to set-up a Swords Dance first.
Team Flare: Neutral. However, The Dark-Types, late-game, are a piece of cake with Close Combat
Elite 4: Neutral against everyone. Return is your best bet, use your coverage moves when needed. Stay away from Fighting Types, like Hawkucha.
Additional Comments: Zangoose has a odd exp. rate (Erratic Group). It is a bit hard to raise early-game but mid-game and mainly late-game he's easier to raise than your other Pokes. I think it's B-Tier because while useful until the end (The harder match-up is Korrina) its typing is average, and those defenses aren't helping either.