For pvp, you're probably better off with each spell on its own key.
That way, your Flame Shock key won't do anything but Flame Shock.

Alternatively, you can set a focus for all of your shammies.
And have your kill button something like:

#show Elemental Mastery
/castsequence [target=focus] Elemental Mastery, Flame Shock, Lava Burst, Chain Lightning, Frost Shock