Personally, the method I find to be most effective is the friendship enhancing berries: all 6 friendship raising berries can be found in the berry pile on Route 10. After that, you take the friendship berries you have, and plant and grow them in Poke Pelago until you have a large enough quantity (50 is probably best). Then, feed them to your Pichu until its friendship can go up no more, and level it up!
Hope I could help!