I did consider doing a hybrid Mythic in which when the players Ascend (become Mythic), they gain only one ability: the Tier 10 Immortality (ie, if they die, they come back 24 hours later). This would provide the players with a sense of being special while also letting players not worry about overpowered foes.
If you allow the players a 25-point-build to start, are liberal with Hero Points, and don't use the Mythic extra abilities, you can otherwise leave the module as-written and they will likely survive. I might recommend doing some trial combats to see if your group will survive.
Finally, there is one other way to tip the scales so it benefits the players: run a GMPC. Anevia would work well for this, as you can give her the Rogue talent to auto-detect traps, and thus not use player knowledge. Thus you have a thief in the party, an extra body to help increase the players' action economy, and a source of trap detection for the group that isn't necessarily fool-proof.
Another GM ran the campaign with no mythic powers for the PC's, but instead he gave them an extra level for every two tiers they had achieved. Of course they wouldn't be able to go over level 20 in their primary class, but they could take five levels in other classes and they would reach their primary classes higher levels quite faster if they wanted to. He had to do only minor adjustments to the statblocks of the opposition as presented in the official AP.
Thanks for the ideas! I think I'll just run it as is with 1 regular level given to the PCs for each 2 mythic tiers and see how it goes. The GMPC idea is also a good one.
One thought. From what I heard, Maguses depend a lot on damaging spells, especially shocking grasp. Since this adventure path has so many demons who are immune to lightning and resistant to other energy types as well as having high SR, should I tell the Magus to play another character class?
If you want to change things up, I had to make a similar change (In our lore the children of a Lightning God were the defenders of the world against demons, so I had trouble pitting that against universal electricity immunity). In my campaign, demons had variable energy resistance (being chaotic). If there wasn't a clear reason for an immunity (ie Brimoriak's Fire immunity), I rolled randomly (1: fire, 2: electricity, 3: acid, 4: cold) for each immunity they had, and that became the individual demon's immunity. Occasionally that meant a lightning attack wouldn't work, but it made the world feasible.