I haven't played SM yet in 6th ed, as to date I've focused on eldar, but I have known a necron army to eviscerate all comers in a recent tournament where their flyers had the run of the table. Did you face a necron air force? My impression so far is that air power is often dominant under the 6th ed rules. He who controls the skies tends to prevail now..... so storm talons and/or flakk missiles (if you and your gaming community have come to conclude they are allowed for missile launchers) would be useful.
Generally, against necrons you want to take away their speed units as fast as possible, from as great a range as possible, and if you still feel the desire to assault them, then assault SMs would be best, if only to get to grips faster.
See if you can get one of your gaming buddies to use their necron army in a one-off game against you. Lessons learned that way often go a long way. If you have a speed and long-range firepower advantage over necrons, then exploit that mercilessly! Also, be prepared to use something like cannon fodder or sacrificial, relatively cheap units to draw necron attention while you press home elsewhere whatever advantage you have. Tanks are too easily destroyed by necron armies, but fast units with long-range firepower like storm talons and typhoons seem so far to fair better and are more useful and threatening.
Last edited by big rob; 08-13-12 at 04:21 PM.