to preform the Shadow-combo, you need to have specced something like this:
(Basicly the best mage pvp specc). When you are done with that you make a macro like this:
/Cast Summon Water Elemental
Now, click the marco once to summon the elemental and once more to cast the freeze, now waht you want to do is, click it once and start casting highest rank of your frostbolt, when you are halfway done casting click the Macro again and freeze the target, now after the frostbolt is done casting you quickly lounch your ice lance. Now this will give tons of burst dmg in pvp.
-Many of you might know this already but just posting this for people who dont know its yet.