Necrons need big squads as their biggest weakness is the phase out. No point only fielding a few warriors bring the rain and field loads. They're as hard as marines and as resilient as a nob with a painboy at his side.
They're not fighters and Necrons have minimal combat units. Those they do have are either pricey in points or can only be fielded in small units so what does that tell you?...don't get in combat. To avoid combat. Stand back and shoot.. Make the enemy do all the work and force them towards you.
With destroyers you have the opportunity to volley long range fire which means the enemy is stuffed if they think they can stand back as well. They're in your range and with the we'll be back roll you'll win any attempt at a battle of attrition based on shooting only. To deal with you they will have to come for you and try and take you out in combat. As they reluctantly storm the field you use rapid fire, canons, monoliths and the Gauss technology to pick off the vehicles that close on you the quickest.
By the time things charge you, you should have reduced the squad sizes and destroyed their vehicle support. In combat you should outnumber most squads which will give you a slight advantage that should hopefully at least even the odds. Not in every case mind...
You need to be patient with Necrons and not get itchy fists. Combat is not an option really. You must have a large core of warriors which means you have to shoot and if necessary fall back a little to buy yourself time and stop enemy squads charging you.
In objective games they will work the best as they sit on objectives and you use spiders and Wraiths as support in the rear until they are needed. Destroyers act as gun platforms. Fire and manoeuvre.