First off, frequently check their targets as they are bouncing around annoying you. Eventually they will all zero in on one guy and begin the rush. You KNOW who they are going to go for at this time. Your shaman needs to do several things in the first couple seconds and his decision making has to be FAST.

1. If he was purging before they rush, and if his spell dmg is high enough he might be able to bust a mage with his EM/NS combo. Before launching he should be in shock range to finish the job. If you pull this kill off you've almost won. That damn mage and his pet eat through your groundings really fast, so with the mage out of the way the shaman's dps can be prevented with groundings and you can work on him next. (as a side note, it typically only takes one volley to kill the pet, so if the mage isn't gonna go down if you can find the time to kill the pet that will help ensure your grounding totems are absorbing big hitting spells from the shaman and mage and not the pet's bolts)

2. If you don't feel like gambling on that insta-gib, then when you see their heroism/blood lust come up your shamans need to PURGE it off of his kill target and if he wnats to switch targets then PURGE it off the next guy. More than likely their DPS will be too high for your shaman to go around purging blood lust off of all 5 of their members so you'll just need to purge it off the first guy and your shaman will have to help you heal.

Basically what will happen is you'll start the match on offense, (1) or defense (2).

If you've taken down one of their DPS early then it should be sooo much easier for you to heal through and your shaman can start mopping up. But if you're forced into defense then your shaman will need to be healing AND putting pressure on them AND purging. It's really mana intensive and he can't mess up or you'll both be outta mana and they will wear you down. On top of all of that, he'll need to keep grounding totems down as long as the mage/shaman is alive and find time to work with his fire totems to keep dps pressure on the warrior.

I used to go for the warriors first, but in a high dps combo like this sometimes i'll start on defensive and scramble to find time to purge a secondary target. Once I get the target softened up i'll switch, throw one last volley of heals on their focus fire target, then try to do the lightning bolt / insta gib combo or just straight up insta gib. You can usually knock a mage or paladin out of action by surprising him.

if the warrior is acting scared and has his shield up you can usually ignore him. The hunter is also doing the healing debuff so killing the warrior at the beginning won't help much. If the warrior is ballz out with his 2 hander eating away your shamans life, then you'll have to think about taking him out. (fire nova totems timed with about anything (shocks, lightning bolt, em/ns) will put him down)

Don't forget your stuns to help keep the pressure off if any one comes in range.

on a side note, your shaman should bust all 4 bloodlusts/heroisms and force their priest into dispelling. It'll take the priest several global cooldowns to keep that many blood lusts off so the priest isn't healing.... This can be a good opportunity to switch up the kill target and bust the paladin because trust me, he's under so much stress being the sole healer than he's not paying attention to a volley coming his way followed by your nuke.

There are lots of ways to take these guys on, there isn't a set kill order in my opinion for this comp.

What usually loses me games against this team is the res'ing, not the actual start of the battle. (i'm playing at 1840 rating right now)