I think the problem of solo encounters is a low level one, and a PFS one.
Use DM Fiat.
Imagine a fight against a blood based demon lord in her personal throne room. You waded through her minions a minute ago, burning a couple spells and some item charges. She clearly has a custom Unhallow, cast from the mcguffin the PC's want to destroy, that grants her extra saves vs ongoing effects and one extra move action a round. Of course she has Wall of Blood as a move action at will.
Being a demon lord, Wall of Blood is a custom spell that acts as a wall of force for 3 rounds before it detonates in a 30 foot (on each side of the wall) cone that deals split force/unholy damage and bull-rushes at +50 cmb.
The Demoness is immune to her own walls.
There. Tack on solid saves and AC, a juicy health pool, a full attack full of bleed effects and you have a dastardly encounter that will rapidly fill with kill boxes of bloodwalls that throw the party around. Perhaps force them to combat heal to wipe out stacking bleeds. Ensures that the one wizard with his 40 DC save or die wont immediately end the encounter. If the PC's reseached even a little for this BBEG fight it wont even be a "rocks fall everyone dies" bs encounter.
This inst a static game that you can't change. Come up with creative and reasonable encounters that rock.