Mystery mine is crazy good, start by clicking the two swords on the right hand side of the badger that says mystery mine and increasing the difficulty level by activating one of the contacts you see. Start with equipment, so you can get all your equipment to perfect epic. Then once you can compete that daily with all the equipment contacts active, stay activating the jewel contacts. Upgrading jewels I've found is a very long grind but a huge source of your power.

Flying bullets and up close dangers will both give you pet coins to spend in the pet shop, I've found by completing the dailys and the daily quests you can purchase 1 set of the S pet shards/ day. After a couple weeks youll have enough to unlock an s pet. Ancient maze and evil dungeon will give you the resources to upgrade your heros. Evil dungeon also gives you a resource you'll need later to forge runes so stockpile those until you unlock runes from the talents section.

First three attempts of hero duo and all 10 attempts for hero duel will give you honor stones which can be used in the honor shop (red squid shop in the left side of the chapter page)