The last boss in ramparts can be really easy for a shaman group. You should be able to do this with your group. I run with 5 shamans but I use one wrath of air totem so I only have 4 grounding totems. Its pretty much the strat on the wiki for 5 shaman group.

Start by pulling the first 2 guys back up the ramp so you dont agro the orc. After that stay in range of your grounding totems and you wont be hit by the fireballs. I normally (slowly) walk my way down the ramp based on my grounding totem cool down.

Once at the bottom I just stand in a corner and slowly dps down the dragon. If you are having mana problems you can just sit here and regen. You taken no damage and dont have to move because of the grounding totems. You can move around with the dragon and dps faster however I just take it slow.

When the dragon is around 35% blow all of your trinkets. At that point he lands and I use the earth totems. I think you should be able to use your pally taunt and finish off his last 20% or so.