I would use banshees. Range is not as good against them because of the shroud so sneaking in or using star engines to get in with fast power weapons will counter the terminators and most of the infantry. use a full 10 man squad. back it up with wraithguard, wraithlords and a farseer and you should be ok. Hit them first and don't let them have armor.
As a Grey Knights player, shoot them as much as possible before they get to you. With the new rules it is very hard to beat them. Blow up any transports and make sure to take out paladins, special characters and terminators.
I personally beat Eldar by careful choice of targets and forcing leadership and pinning test. I use Tau Empire so I am able to use heavy firepower to remove the Eldar's primary advantage: speed. Their vehicles are very good and require significant firepower to remove. A few sniper shots and large explosions after that seals victory.
You can defeat the Necrons as eldar if you send in Taldeer with many Fire Dragons to use the bomb, make sure Taldeer has a bodyguard.
Nobody is going to beat those knights. They are unstoppable beasts that get krunk like no other. So we are the best.
no, there is not
you beat the island
You can't. You must beat all three planets and take the three knights to the giant asteroid. (see the related question and try again)
Yes
one
yep
all you get is the island medallion and 100 credits
well just beat the stupid thing ingnore her baby birdies