Heroes to invest

Don’t invest much in Artemis (only good for Outland bosses)
Thea is only healer for early to mid stages of the game, so keep her leveled up. Switch to Martha, Celeste, Maya, or Dorian as soon as you can.
If you can’t get healers mentioned above and do not plan to spend some money on leveling them. Thea is good a choice for a late game then and is worth investing.
Soul stone missions are important – farm them to earn that hero’s stones!
Don’t farms for Phobos, Fox, Mojo, or Heidi – they’re not worth it
Ginger is only hero worth farming stones from the early chapters
Galahad is a good tank and damage dealer, highly recommended to invest in him Especially for F2P players
Earn Astaroth through quest rewards – he’s one of the top tanks in game, but takes time to upgrade
Pair Astaroth with Galahad – they’re a great combination
Phobos has stun which is good for bosses but nothing else, don’t invest heavily
Arachne is a trap, don’t invest in her, there are a lot better mages
Prioritize investments to tanks and healers and only then focus damage heroes
Chapter 3 has soul stones for Keira, it is highly recommended to farm these
You have to decide if you want to farm Ginger’s or Keira’s stones, so you have to pick one (At the current state of the game, Keira is way better)
Make sure to farm Astaroth and Galahad soul stones in Chapter 5

That’s it for now, if you found this useful, please let us know so we will keep adding tips.