In pathfinder gameplay deteriorates after 17th level, so I came up with an alternate system:
In stead of progressing past 17th level you earn:
Combat Points: You earn combat points when the Gm says you do. You can spend combat points for:
An ability gained from an archetype or class- costs 2 combat points per level the archetype gains/ upgrades the ability at. ( but you need evasion for imp.evasion etc.)
1 point abilities:
your CON mod in bonus hit points.
1 extra spell per day
More later:
If you are not aware of them, you should look up "Pathfinder E6" on google. Many people have similar rules, just at a much lower level. E6 seems to be the most common (ending at 6th level), we tend to play either E8 or E12 personally.
In 17th the game is beyond saving. E6, E8 and E10 works best.
That, or try Spheres of Power, for a way better casting system, because Martials can't break anything.