So what have I paired my Eldar with lately? Lets look at this idea. Dark Reapers......... with guide and fortune sitting in a second level ruin has proved to be devastating on the field of battle. I was playing with my Imperial Guard, so destroying tanks was not an issue. For troops, I simply used a single squad of 5 rangers.
The list is very short on what cannot pair up with Eldar.... Chaos Daemons, Chaos Space Marines, Necrons, and Tyranids. Everyone else that is not interested in dealing with psykers can simply take a small allied detachment of Eldar. Not sure why this is?...... here is the scoop for those that do not know...
with Runes of Warding...
A farseer can throw up psychic inerference to hinder its foe. All enemy psyhic tests (careful here if you are allies of convience and have other psykers) must be taken on 3d6, suffering perils of the warp attack on any roll of a 12 or more.
Farseer 55pts, runes of warding 15pts. Of course you will probably want to add in Fortune and Guide, and possibly some wargear, but overall the cost of a Farseer is very inexpensive and his value is very high.
Playing Dark Eldar who have no psychic defense or powers...... Enslave yourself a Farseer, in fact, put him in chains for a fun conversion, and throw those midguided craftworld kin at the enemy. I am sure your Haemonculi will enjoy their new toy. (you are battle brothers, so taking some eldar for strong psychic powers and effects is a huge advantage).
Tau..... No psykers in your army either.... Throw in your Eldar and your good. For Tau, eldar are battlebrothers as well. So lucky you... some strong psyker options in your army.
Imperial Guard. Also no real psychic defense until you throw in one farseer. Then its a psychic battlefield for you and your troops, under the watchful gaze of a farseer.