In this Misty Valley guide, I will recommend formations that don't include Celestial, Hypogean, and Dimensional heroes unless necessary or a Silver Challenge requirement. I hope it will help you complete all 60 challenges and unlock all 60 reward chests.
If you need more formation recommendations for a particular stage, feel free to leave a comment with your available heroes and I will try to help you.
In cases where there was a 4-faction requirement, I tried to use 5 heroes of the same faction. Just replace one with your main carry.
When the enemy formation contains Kren try to kill him fast as he hits very hard.
For stages where one hero needs to kill 3 or 4 characters, I recommend you give them the Wrizz and Soren treatment with a 1+4 setup. Let Saurus do the killing and surround him with 4 support heroes tanky enough to not go down fast and let Saurus be overwhelmed. If your Saurus is SI +30 then these two stages should be piece of cake.
Note: This is the stage 1-10 guide for September's Misty Valley event.