Speaking of which... I just defeated Korrina! Here's my team upon defeating her.
Berserk- Male- Level 32
Adamant- Capable of taking hits
Overgrow
Poison Jab/Leech Seed/Needle Arm/Hone Claws
Orca- Female- Level 32
Docile- Mischievous
Huge Power
Return/Aqua Tail/Double Edge/Aqua Ring
(Thankfully, I'm not as lost with Orca's moveset now that she has Aqua Ring.)
Buzz Buzz- Female- Level 32
Brave- Scatters things often
Pressure
Defend Order/Confuse Ray/Heal Order/Slash
Samuel- Male- Level 32
Hasty- Good perseverance
Blaze
Fire Fang/Dragon Rage/Flame Burst/Smokescreen
Hamlet- Male- Level 35
Timid- Good perseverance
Own Tempo
Confuse Ray/Power Gem/Rest/Psybeam
(I'm half-considering getting rid of Confuse Ray or Rest later. They fit, but I have other options for Confuse Ray, and though using a ChestoRest strategy might be good to restore HP when I need it, I don't know where I'd get a Chesto Berry.)
Gallows- Female- Level 32
Modest- Mischievous
Flash Fire
Flame Burst/Night Shade/Hex/Will-O-Wisp
I'm assuming by 'defeating Korrina', you meant the Gym battle itself, so I'll be leaving the whole Mega Evolution thing until the next part.