There are a few tips I can give you.
1. Level up your Pokemon some more
I know you said you should do this, but I figure I should state it anyway. If you don't have the Exp. Share turned on, you should definitely do that. You should try putting one of your lower level Pokemon at the front of your party and then, when you run into a wild Pokemon, switching out for Primarina so it can battle. Your Pokemon will get more exp. points that way.
2. Make sure your Pokemon are holding useful items
Make sure your Pokemon are holding useful items like a Z-crystal or berry to eat when their HP is low. That way, they can attack with more power or be able to battle longer.
3. Be prepared for the battle
Make sure that you have a lot of Revives and Super Potions so when your Pokemon's HP is low, you can restore it. It might also be a good idea to have an X. Sp. Attack at hand.
Beating totem Mimikyu
When battling totem Mimikyu, make sure that you beat it first and not be distracted by the Pokemon it calls. If you beat Mimikyu first, it will be easier to beat the Pokemon it calls later.
I hope this information helps! :)