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Necrons are a serious threat, just like Tau, but I managed to defeat them with Space Marines. The simplest, yet most effective tactic is to use mass Boltgun units supported by Vortex units. The infantry with mass Boltguns prevents the Necrons from getting close, and the Vortex units help with that. The only problem is that the Necrons arrive at the base around the 4th minute, while it takes about 7 minutes to develop at least to the point of having Boltgun units. Scouts can help, but it's still difficult. You also need to pay close attention to the battle to prevent the Necrons from using artillery, otherwise, it's a disaster.
Next, I tried using mass Glider units. They are relatively cheap, and their combat power is good, but you need to use them skillfully.
Don't bash Eldar; they are not an anti-Imperial faction. I easily defeated Tau with them. You just need to know how to play them. However, I can't seem to play as the Imperials; it just doesn't work, although it's an interesting faction. But Space Marines tear them apart with ease.