eldar
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.
AnswerImperial Guard Eldar and Tau Chaos, Necrons, Orks, Tyranids and Dark Eldar are considered the "bad guys".
In warhammer terms TAU is a young yet sophisticated army which includes kroots and fire units. Personally I believe Tau are the best with Eldar in warhammer 40k.
AnswerIf the Tau are fighting Eldar you should try to out range them with fire warriors and take out any falcons or avatars with rail guns (broad sides work best) and do not let them assault you. If this is a problem take Kroot they add good fire power and are ok at assaulting but if you are fighting Eldar you have to watch for devastators and rangers other than that you should be able to out shoot just about anything but your first targets should be storm guardian's striking scorpions, howling banshees etc which ever has the highest initiative.The best way to find out is to fight an Eldar player even if you lose it will be a good experience but the Eldar are very diverse and unpredictable but the Tau have many units with jump packs so try out maneuvering them and kill there precious assaulters.
Try to stay in stealth. The Tau have got some stealth of their own though, and also, don't fight them in head to head combat. As Dark Eldar, try and fight dirty.
You must have the previous games to play the other races. With the Original you get the Space Marines, Eldar, Chaos and the Orks. Winter Assault you get Imperial guard. With Soulstorm you get the Sisters of Battle and the Dark Eldar.
Eldar; Use lots of Howling Banshees. Tau; Groups of Fire warriors and Commander Shadowsun Imperial guard; Keep 5 Basilisks behind the lines and keep shooting across the map.
In the original game, without expansion packs (Warhammer 40,000: Dawn of War), you have the option to play as the Orks, Space Marines, Chaos Space Marines, and Eldar. If you add the expansion pack "Winter Assault" (requires Warhammer 40,000: Dawn of War or one of its standalone expansion to play) then you gain access to the Imperial Guard. The standalone expansion, "Dark Crusade" (does not require Warhammer 40,000: Dawn of War, or Winter Assault to play, but you can only access armies which come with the game) 2 new armies, the Necrons and the Tau, are added. Finally, in "Soulstorm" (another standalone expansion which doesn't require Warhammer 40,000 or any of its expansions to play) adds the Dark Eldar army and the Sisters of Battle. Warhammer 40,000: Dawn of War is stand-alone. Its expansions, Dark Crusade and Soulstorm, are also standalone (Winter Assault is NOT standalone and requires the original Warhammer 40,000: Dawn of War game or one of its other expansions to play). However, you only get armies that you pay for. For example, buying only Dark Crusade and Soulstorm provides you with only the Necrons, Tau, Dark Eldar, and Sisters of Battle. You cannot access the Eldar, Space Marines, Orks, or Chaos Marines, who require Warhammer 40,000 original to play.
Eldar Space Marines Imperial Guard Dark Eldar Tau Chaos Space Marines Witch Hunters Gray knights Chaos Daemons Necrons Orks Tyranids
I think it really depends on how many you have or how much damage they do
Space marines Chaos space marines Necrons Orks Tau Space wolves Daemon hunters Chaos daemons Eldar Grey Knights Tyranids Blood Angles Dark Angles Dark Eldar
Dawn of War featured the Space Marine, Chaos Space Marine. Orks and Eldar Armies. Follow Up Expansions Featured the Imperial Gaurd, Tau, Necrons and Dark Eldar. Dawn of War II Features the Space Marines, Orks, Eldar and Tyrannid Armies