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my tactic was used in a real life battle between 10,000 romans and the massed forces (100,000 britsh tribes) of boudica (i dont know if this is the correct spelling) the romans won in the battle that followed, i adopted the tactic the romans used, it hasn't failed me so far.
what i did was place the slow moving units (the FW and vehicles) in organised columns and left my battlesuits and stealthsuits in reserve.
the romans used thhis tactic also, the legionaires in organised formation, the chariots go to the flanks.
turn 1: nothing really happened, my hammerhead fired and stunned the one of the enemies vehicles.
ancient terms: the romans stay in formation, the chariots still moving around the flanks
turn 2: my battlesuits deepstrike to the left, the stealthsuits outflank to the right, enemy casualties from the XV's a tank squadron, command squad, immobilized the monolith.
ancient terms: the chariots charge in cutting through the tribesmen
turn 3: more killing,
ancient terms: the same
turn 4: my battlesuits retreat to the safety of my gunline
ancient terms: the chatiots return to the flanks of the roman formation
turn 5: the enemy phases out
ancient terms: the enemy runs

conclution: this actually worked, continue using the tactic untill failure

Necrons have 24" guns, 36" max with destroyers. I'm assuming you were playing around 1000 points, and since he used a monolith, he probably didn't have much else.
You staying outside of his range but still being within killing range of your guns won you that one game.

I would watch out for any opponent with superior range or speed to your suits. Anyone with some mechanization will drive over to your suits and smoke them, then focus on taking apart your column. Hordes might as well ignore the few casualties your suits do at range and murder them if they come close to use flamers. And god help you if you face a real gunline.

Well, 40k can be described by Lanchester's square law.
Please have a look at Wikipedia.