well against your dark eldar friend i cant offer much help. ive never fought one and i doubt i ever will.
the space marines on the other hand are different.
let the marines get into rapid fire range. shoot them with the double shots and then use the monolith to teleport them out of the way.
another strategy is what ive already said about the wraiths.
also make sure your lord(s) have ressurection orb and keep them close to your warriors as they are your weak point and your best strenght. also if you see any powerfist or power weapons in your enemies ranks destroy that squad. thats one of the few weapons that will take a necron out of the game for good.
also a word of rule advice. if your throwing out more shots than he has squad members you can designate one shot to anyone you want. if he has 10 soldiers and you have 11 you can designate one shot to that sgt of his and attempt to take out the power weapons/fist
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