Eldar Ranger/Pathfinder Tactics
Rangers are one of four Troops choices that a Codex: Eldar army. And at 19 points per model, they are by no means a cheap unit for a unit that is armed with (special) sniper rifles and has stealth, MTC and infiltrate. On the other hand, they are an undeniably effective unit when used correctly, with AP1 on both a 6 to hit (which also allocates in 6th edition), and to wound, combined with the regular abilities of snipers to wound and to damage vehicles.
Wargear and Stats
Rangers don't have much wargear by default, armed with a Ranger Long Rifle and a Shuriken Pistol, the Long Rifle being the point of interest. It has Sniper, which is effectively Poisoned(4+) and Rending. But what is even better is the way the 'Rifle works. On the roll of a 6 to hit, the shot becomes AP1, and can allocate to wound to a model of the shooters choosing. This makes it effective at picking off weapons that ignore cover and heavy weapons etc. This is good for picking off weapons that can damage your other units, like lascannons or broodlords. They also have the Masters of Stealth special rule, which grants Stealth, Move Through Cover and Infiltrate. Stealth grants you a 3+ cover save when deployed in a ruin, making cover a must for rangers. Infiltrate is useful for getting into those tall buildings just outside your deployment zone. Move Through Cover isn't particularly useful, but handy nonetheless. The statline of a ranger isn't very good, with only I4 (which is rare for Eldar) but BS4 is great for what you want to do with them. But if your Rangers are getting into combat, you're doing it wrong.
In order to make the most out of your Rangers, you need to deploy them well. Infiltrating into a building 18" away from your enemy is NOT something you should do. Deploy them in cover, preferably in a ruin or building where they can get line of sight to the majority of the board. Try to put them into a position where they can be supported by your other units, such as Dire Avengers, Guardians and Support weapons.
Rangers are best used against high-value targets with sergeants and heavy weapons, such as Devastators, Terminators, Noise Marines, Sentinels, Sternguard (I'll mention these guys later) and so on. Remember that they don't have a high number of shots, and can also pin down enemy units. Keep close combat units and flamers away from them, and make sure you take out the greatest threats to them first. After all, a unit at half strength can put out now where near the same amount of damage as a unit at full strength. After taking out threats to them, take out what will be crucial to the mission's objectives. This ensures that enemy Troops are obliterated, making it very hard for them to win. When playing The Relic, always target enemy relic bearers. If they get away with it, you'll have no choice but to take it back.
Also never charge your rangers into assault. If they are about to get assaulted, try to use their final shots on something of value-Dying to overwatch will not win you the game, while your overwatch can allocate and is also AP1. If you can use Guide and Fortune on ranger units, and use Doom on their target, to get the most out of this unit. Keep in mind that you can also claim objectives with these guys. Sternguard are also deadly, with their boltguns that rip through cover and demolish your armour. Kill them on sight!
Of course, I can't help but mention Pathfinders. They operate like regular rangers, except they cost 24 points per model, and gain the Pathfinders special rule. The Pathfinders special rule grants you Stealth (+2) Scouts and makes their Long Rifles AP1 on a 5 or 6. This means that in a ruin, they have a 2+ cover save, and when Fortuned and Guided, make a (very) scary unit. I usually run two farseers, one with Guide/Fortune and the other with Doom and another power, and attach the Seer with guide to the Pathfinders (or keep him close). Scout also grants you a 6" redeployment.
So to conclude, Rangers/Pathfinders are an excellent Troops unit, albeit expensive. From experience, I have yet to lose a single Pathfinder to shooting, making these guys an excellent fire support unit. They are also Troops, so sit them on an objective and blast away. (Absolutely hilarious if you roll skyfire nexus) Just make sure to keep any flamers/close combatants out of arms reach and these guys will win you games.