Gym #1 - Viola (Santalune City, Bug-type): Just don't. You might be able to take down Surskit with little problems, but Vivillon has super effective Infestation that you cannot switch out of. Use something else.
Professor Sycamore (Lumiose City): Squirtle can go down in a few Vine Whips, but avoid the other two. Bulbasaur has decent defenses and Charmander has Ember.
Tierno (Route 5): This match is probably the easiest so far for Pansage if it is highly enough leveled. Just Vine Whip Corphish into oblivion.
Tierno and Trevor (Route 7, tag battle with rival): You should still be able to take down Corphish and Pikachu has only Quick Attack to hurt you, but watch out for Static. Flabebe should be no problem though, especially if Corphish is gone.
Gym #2 - Grant (Cyllage City, Rock-type): Even though Pansage/Simisage is a Grass type, Amaura has STAB Take Down thanks to its ability and Tyrunt is powerful and resists your STAB. Especially avoid the battle if you have not evolved yet, as both are too powerful for Pansage's defenses.
Korrina (Geosenge Town, Fighting-type):
Rival (Tower of Mastery):
Gym #3 - Korrina (Shalour City, Fighting-type):
Rival (Coumarine City):
Gym #4 - Ramos (Coumarine City, Grass-type):
Aliana (Kalos Power Plant):
Gym #5 - Clemont (Lumiose City, Electric-type):
Rival (Route 14):
Gym #6 - Valerie (Laverre City, Fairy-type):
Celosia and Bryony (Poké Ball Factory, tag battle with rival):
Mable (Frost Cavern):
Rival (Anistar City):
Gym #7 - Olympia (Anistar City, Psychic-type):
Lysandre battle #1 (Lysandre Labs):
Aliana (Lysandre Labs):
Celosia (Lysandre Labs):
Bryony (Lysandre Labs):
Mable (Lysandre Labs):
Xerosic (Lysandre Labs):
Lysandre battle #2 (Team Flare Secret HQ):
Xerneas (Team Flare Secret HQ, X only):
Yveltal (Team Flare Secret HQ, Y only):
Lysandre battle #3 (Team Flare Secret HQ):
Professor Sycamore (Couriway Town):
Shauna (Route 19):
Tierno (Route 19):
Trevor (Route 19):
Gym #8 - Wulfric (Snowbelle City, Ice-type):
Rival (Victory Road):
Elite Four Wikstrom (Pokémon League, Ironworks Chamber, Steel-type):
Elite Four Malva (Pokémon League, Blazing Chamber, Fire-type):
Elite Four Drasna (Pokémon League, Dragonmark Chamber, Dragon-type):
Elite Four Siebold (Pokémon League, Flood Chamber, Water-type):
Champion Diantha (Pokémon League, Radiant Chamber):