I agree with what ElectronDF said. Try to heal through his initial burst damage (i found this would be the strategy against all melees who jump you team). The DK would usually put the anti-magic shell up first when he sees this many shamans. It reduces incoming spell damage by 75% for 3 seconds. So don't burn your CDs when you see this green shell surrounding the DK. Wait for the shelll down, rotate Thunderstorm, + CL or LB and shocks, he should be dead.

I first had problem when they death grip my main when I had no totems down. But by healing, no way he can kill any of your shamans when 4 others are healing.

Edit: didn't see you have 4 shamans, not 5. I would suggest take all the damage reduction talents under the elemental tree, don't really need the extra damage & regen because you can 1-2 shot all the mobs. So survival > everything else. If you are like me, I get hella annoyed if my team get ganked. Nothing feels worse than putting this much time and effort, still gets pwned. So I just do whatever I can to prevent that from happening.