When I was levelling my shammies I must admit they were pretty useless until level 20. Once they got the healing totems and lesser healing wave they totally transformed into madmen and could do any equal level instances in Azeroth (Outlands instances can be different in places like the last boss in Ramparts etc).

My general tactic is similar to Nepdia (but I use 5 healing totems and a slightly different totem setup) and just nuke like crazy on a single target and heal with all shammies using lesser healing wave on a single shammy. The initial target should be dead before it reaches you but you have to be quick with the lesser healing if one shammie has all the aggro. Once the mobs reach you make sure you prioritise healing over dps and you should be fine. All my shammies are 41/0/20 for PVE and 40/0/21 if I go to BG. Only problem I found in the early levels was Mana but this gets better as you hit late 40s.