If the sniper is able to hide after making the shot, his opponents should have ot make a spot check against his new hide check to shoot back at him, even with a readied action. If all you needed to do to attack someone well hidden that you can't possibly spot was ready an action, the whole hide/spot mechanic falls apart. Note that the default rule is a -20 on Hide after sniping, so it's usually not
that easy to do.
And you do need to hide in a place that is viable, such as an area that gives you cover from view. If you're staying in one spot the whole time, the party will be able to figure out the general area shots are being fired from and close in around there. If you have the ability to hop hiding spot to hiding spot, most areas will only have a limited number of good sniper perches. If the area you're in happens to have a great number or near unlimited amount of easily accesible sniper positions AND the sniper has the ability to evade your spot checks even with the sniping penalty to hide AND has the ability to seemlessly jump from one area to another...GET THE




OUT OF THERE!!!!!!
Sorry if that's not helpful to you, but seriously. Pick your field of battle. When an enemy has such overwhelming terrain advantage over you, and you choose to stay (or worse, were baited in due to foolishness and have had escape routes cut off), you deserve to get your ass kicked.
If not all of those conditions are true, then the problem can be dealt with by mundane methods as have been described, such as fanning out over the room to smash things up until you ruin his hiding (you can blindly swing into squares if you think an invisible foe might be there and hope to get lucky, so something similar, but with more property destruction to ruin his cover or otherwise flush him out). Or firebombing the whole area you think he's in and asking questions later. Or taking a defensive position such that you have cover from attack in certain directions, forcing him to "reveal" himself if he wants to attack you further -- if you can deduce the areas an attack can only come from, that greatly helps in narrowing down the little weasel's location.