Some stories involve characters who are relatively low level but whose mythic gifts allow them to overcome challenges that would normally be beyond their capabilities. When you want to tell these kinds of stories, you'll need to make the following adjustments.

As your characters grow in mythic power, their endurance and other capabilities will grow, but their ability to deal damage will only increase if they choose mythic feats and options that increase their attacking power, such as Mythic Casting or Mythic Strike. As such, rather than throwing higher level enemies at the characters, you should first adjust the difficulty of the encounters you are using, starting by removing low-threat encounters and using a larger number of moderate- or severe-threat encounters. If the PCs are handling even severe-threat encounters with relative ease, test their capabilities with an extreme-threat encounter. Only after the PCs have successfully overcome at least a couple extreme-threat encounters should you consider treating them as a higher level for the purposes of encounter building.

You can use a loose rule of thumb that says every three mythic tiers the PCs possess is worth the equivalent power of about one level, but this can vary widely based on party composition and the choices they players make when leveling up their mythic characters.