If you are at level 15 on the base, you need to get a lot of coins so that you can craft a lot of power unit kits in the item factories. They have a lot of boost power and can help you to upgrade stuff quickly.
You can get a lot of tokens easily by doing daily missions, campaign, participating in raids, leveling up, etc. Also you can try saving the tokens for later, because sometimes there are offers that can get you 15 Premium Packs for only 2000 tokens. Those offers give you 15 of them for the price of about 6.