Restaurant Le Wow. Triple/Rotation battles. Exp O-power, Lucky Egg. Pokemon Amie Affection min level 2 for Pokemon you're battling with.
It'll skyrocket quickly and Pokemon are healed between battles. If you have 2 really weak Pokemon and 1 strong, go with Rotation. Weak Pokemon in 2nd & 3rd place. Strong in 1st. Have Strong 1 shot Pokemon and weak pokemon in the side slots will rocket up.
If all 3 Pokemon are at decent levels switch to Triple Battles to kill Pokemon faster. Protect will stall you a bit but the best way to do it to me. Also not as boring.
Donnel is my # 1 bro. We fist bump, slay some risen, eat some chicken strips, bomb the Valmese and steal sugar from Gaius on a daily basis. Like a Boss. FE:A