So I havn't played any of the actual board game
So I am going to ask how the Necrons "You can't actually kill their units" is reflected in the game.
Are there rules for skirmishes and for actual "games" that span over multiple battles?
Necrons: Roll D6 on death. 4+ comes back to life. This can change depending on weapon used to kill (doesn't work with power weapons, templates and a few other things) and as long as there are remaining units in the squad the unit is from. There's also some other rules for larger games where AoE regeneration can reduce the roll required.
As for skirmishes and larger wars; not that I'm aware of, but of course players can modify their armies and so on to represent an on going battle. Tank gets destroyed in early battle, may not come back until the third battle if you decide to have multiple.
Most battles in W40k are skirmish levels. Space Marines will have about... 20ish models; 3 squads of 6 troops, a tank or two and a commander. My Necron army at 500 points could only have 20 warriors and 1 commander, 1000 points added 3 destroyer hovers and about 6 melee units. Plus some wargear (items the commander takes, armour and weapons).
The larger games are Apocalypse which can have essentially limitless battle sizes. Something like 5000 points or higher. Allows the usage of titan models which are really huge walkers and the like. Warhound, Reaver, Warlord and Imperitor for the Imperium.