With 5 shammys, I lay down my totem farms, then nuke watcher first (i think it's the watcher - it's the one that runs right up to you and isn't the skirmisher). With LB,CL,FS,LB, he should be dead by the time he reaches your group, the skirmisher will be there at the same time too. TS to keep him at range, then alternating your volleys. Make sure you keep your grounding totems down as the caster in the back won't hurt you all. If you keep using TS to keep the skirmishers at range, you should a short period to do a quick chain heal / drink before the next set comes.

I have no clue how you are running out of mana so quick unless you are chain healing constantly. if mana is a big issue, then make sure to drop mana totems.

My first several attempts on that sound just like yours. Once I started using TS to knock the skirmisher back and keeping the grounding totems down, it became a lot easier.