The shaman starts to shine when you get chain lightning and the ability to get mana regen from the chain lightning (maybe a glyph or a talent for this), I know from running with you in BGs on Mag that when I can use chainlightning spam on 3+ targets I actually regen mana faster than I can use it. For the elemental shammy ensure you are using your thunderstorm/pushback to regen mana each cooldown.. in PVP it is survival technique in PVE it is just regen or for gimick fights.