I've noticed that the big bad guys in high level adventure paths have some powerful spells with VERY high DCs ... 28-30+. 14-16th level PCs have general bonus to saving throws, stat bonuses (if they're high enough), item of resistance and then a buff spell. Even with all of that odds are still stacked against them. Heavily. Not all parties have Bards to buff them ... Besides the occasional divine buff spell or owl's wisdom type deal from an arcane source, odds still seem grim. Curious to hear what everyone does to give their characters saucy odds. Especially with final bosses whipping out greater dispel magics like tic tacs.
I understand one wouldn't want DCs too easy, but if characters HAVE to roll 15-18 every time, it's over faster than Usain Bolt.
Big bosses are supposed to be hard, so I see no fundamental problems.
There are many feats, items etc. that allow a reroll if one failed. Some classes also provide resistances/immunities - this is where these classes shine.
Usually PCs would deal with this kind of opponent by countering his actions, e.g. readying counterspells/dispels, attacking when casting, winning initiative, silence zone on an enemy caster, antimagic fields...
The best defense in Pathfinder is a good offense. Rocket Tag if you will... kill them fast before they kill you fast. PCs also have an advantage in that there tend to be more of them (more actions) than the bad guy gets.
Shut the bad guy down with a single Save or Die spell to their bad save
Dish out a ton of damage fast
etc.
etc.
Let the party paladin draw the first attack. He's got the bonuses to laugh at those save DCs. Assuming you are halfway expecting a fight like this, sticking Silence on your melee character and sending her in close is one way to ruin many casters' day. Their DCs don't matter if they can't get the spells off. Grapples are another way. Of course, at some point, the GM will retaliate in kind, and there ARE defenses against these tactics, bit they ought to work well once or twice at least.
Also, Spellturning is not nearly as fun for them as it is for you if they have DCs that high.
At such levels, the best defense is divination: if you have some idea of what you face, you can prepare countermeasures (spells like spell immunity, greater heroism, etc).
Also, bear in mind that most of these effects are targeted, so if you can't be targeted - greater invisibility plus mind blank, blinding the enemy caster with dirty trick, blindness, power word:blind, fogs, etc - nerf powerful enemy casters greatly.
Forgot ... among its many saucy features ... that Mind Blank provides a +8 bonus to mind affecting spells. That covers all enchantment & domination spells.