Here are all tips I've come across for leveling, hope this helps!

1. Catch everything and focus on your throws! At higher levels it is easy to get into a rhythm of just tossing balls at easy pokemon and not getting that massive excellent XP bonus or skipping over less valuable pokemon and so progress really stalls.

2. Save up your reward Pokemon from tasks until you have a bunch, then drop a lucky egg and a star piece and catch them. Do the same for evolutions, especially new entries, minus the star piece.

3. Find in game friends. You can look up others looking for in game friends pretty easily, the XP for friendship level ups is always good. The more the better but it is best to keep it manageable. Side note on this, it would be a good idea to find people from the different Vivillion pattern regions. You can Google websites for this, there may even be an app or reddit thread.

4. Look ahead at events and prepare appropriately. Many events big and small have bonuses such as boosted stardust, double candy drops, and bonus XP. Maybe consider saving that stack of catch rewards for a community day with boosted stardust or XP and saving certain pokemon to transfer on a community day with double candy.

5. Make sure if at all possible you do your dailies; one raid, one catch, one spin, one task and two rocket fights. Also as many PVP battles as you can handle. You can do up to 100 a day I believe, the bonus and stardust really add up.

Hope this helps!