Well, the Experience Share is very good for leveling up weak Pokémon without having to use up a turn each time the opponent sends out another Pokémon. However, as you may know, Pokémon who only gain experience via the Exp. Share will only get half the experience from battling and will not get boosts, i.e. from being traded or with the Lucky Egg.
But my recommendation would be to just use the Exp. Share. It is far less annoying than having to switch over and over, and without wasting turns, you won't need to use many healing items if your Pokémon are powerful enough.
But if the reason you're going through the Elite Four is to level up your Pokémon, I have a couple of much better ideas.
Method #1: Fast and efficient, gives you money
- Get 3 Pokémon: 1 good one (preferably lv 100) and 2 that need to be trained
- Give the first one an Amulet Coin and give the other two Lucky Eggs
- Go to Cafe le Wow in Lumiose, choose Rotation Battle, and watch the money and experience roll in!
Method #2: Easy Way
- Drop two Pokémon you want to raise off at the Day-Care
- Go to Centrico Plaza in Lumiose City, get on your bike, and stick a coin in the joystick
- Leave it like that overnight, and then pick them back up the next day at around level 75!