Here are a few things to consider :
Tactics
If a full attack is "all" it takes to down someone, force Move actions. Could come from cover/concealment or combat maneuvers like Trip/Knockdown or Disarm. If you just play "rocket tag", you're implicitly accepting that if you miss, there's a good chance you're dead.
Note that this is exactly what happens IRL between powerful forces in a battle and in this respect, is not a "bug" of the system but a simple truth.
Intelligence
So you're targeted by a Master Ninja, who manages to sneak up on you and place a Death attack. Curse you, Hayabusa ! Wait, you made your save ! Time to grab your trusty bow and fill him with arrows...
Well, boo. The ninja has broken/stolen your bow as well. Not only does he make his escape, he's slowed you down by forcing you to have it repaired / replaced while you try to get it back.
This goes a step further in Tactics, and is to be expected of Ninja, for example. Study your target. Know their strengths and weaknesses as well as yours. "That Brilliant Bow of Badassitude could be a problem if I must escape. Can I get rid of it ? What about impairing the bowman ? Maybe I could blind him ?"
As an aside, the GM should make sure the players don't feel cheated though. Extreme competence is assumed as part of the characters (especially past level 10) and robbing them of that can be frustrating to no end for some.
Numbers
So they can take the Big-Ass-Monster down in a round ? What about 10 Medium-Sized-Nuisances ? One by itself could barely hope to hit them, but with flanking, teamwork feats and simply being all over the place, the little pests could prove to be annoying. Add in a Leader-type and you could even be worrying your characters.
Story happens
Take the characters down a notch or two through Story. Have them stripped of Rank and Privileges by the King or even make them straight Outlaws. Have them stranded on an island (and some of their equipment lost to the sea) after their boat got caught in a storm. Have their home base attacked (and their Mentor killed, leaving them unable to progress in their main class) while they were on mission.
Change focus for a while
Once all is said and done, if the combat has become so easy it's boring, it may be time to try a little courtly intrigue for a change. Or why not a mystery ? Possibly meshing with "Story happens" above, there are plenty of opportunities to take the players / characters out of their comfort zone while staying true to the setting. They'll be happy to resume bashing heads once they've stumbled for hours finding a tangible threat to pounce on. Or hey, maybe they'll actually enjoy trading piques with the Jester and decide they want a piece of land and a throne of their own ?
I would argue that Tim's answer is not accurate. I understand that his proposed solution is not based on raw, but it is based on the idea that "your opponent must fight gravity to advance".
That it is just not the case for this rule. Higher ground advantage does not seem to emanate from a difficulty your opponent has, this is already covered on a separated rule for fighting uphill. It seems to come from an ease to strike from having gravity aid you. Swinging down is easier than swinging up.
This is further evidenced by the rule on Combat while Mounted that specifically determines you get high ground bonus while mounted if your target is smaller than your mount.
It is still up to the GM to call what high ground is, I would recommend simply giving higher ground advantage to any creature who is at least 5 foot higher than another.
Best Answer
The Dirty Trick maneuver has some limitations. If your player wants to attempt Dirty Trick frequently, then you should make sure to consistently enforce the rules.
Dirty Trick can only impose temporary penalties. In your player's case, you can say that the target is temporarily blinded, and then may spend a move action to wipe the glass dust out of their face. Or they wait it out. Either way, the enemy is not permanently blinded.
There is no saving throw involved. It's a combat maneuver that the player would attempt versus the enemy's CMD.
Attempting a Dirty Trick normally provokes an attack of opportunity, unless the player's character has the Improved Dirty Trick feat or a similar ability.
Dirty Tricks cannot cause damage. Even though having glass dust in your eyes is probably painful, no damage would be involved if it's done via a Dirty Trick.
If you want the thrown glass powder to cause actual hit point damage, or involve some sort of saving throw, then you may want to treat it as an alchemical weapon instead of a maneuver.
For example, the "Dust Knuckles" item involves throwing dust toward the eyes of your opponent. The target may make a reflex save or be blinded for 1 round. Glass dust sounds very cheap to make, so if you treated it as an alchemical weapon, then I imagine it would have a very low save DC.