My team can still only reliably kill 2 heroic bosses (the mammoth turning one and eck - enter from the far side from the summon stone in gundrak) and that's with a lot of wellfare pvp epics on the shammans (sadly there isn't any wellfare tanking gear I could find so I'm still mostly in crafted blues; not affording yet the crafted purple)
I think as many have said having a resto among your shamans is a must - earth shield alone makes a world of difference (glyph it too)
Because I'm not too successful I'm just doing the Midsummer for cash and distraction and fun (and xp on some lower alts I swap in) but I think with a few weeks of dailies you can afford to craft or buy crafted epic to make your tank good enough