Stephan: Another problem for Tau when fighting Necrons is that Tau anti-vehicle units are not so great at destroying Necron vehicles. The reason is that Broadside Battlesuits move too slowly. They can easily be countered by the Necron Lord Solar Pulse ability. Also, weapon upgraded Stealth Suits die fast due to the fact that their attack range is really short. In order to get around this problem, always research the Missile Launcher upgrade on the Tau Commander, and always target the Necron vehicles (Destroyers first, since they kill Fire Warriors quickly). Using a squad of Path Finders to mark and target the Necron vehicles is also a good idea.
Finally, if the Necron player has a massive vehicle based army, use Broadside Battlesuits, but always remember to spread them out so that only one squad is affected by Solar Pulse. They destroy vehicles faster when not entrenched. If the Necron has a massive infantry army in the late game, build the Kauyon Command Post, instead of the Mont'ka, and purchase both of the Fire Warrior upgrades. Necron infantry shouldn't stand a chance versus them.