Spirit Walkers Grace + Ascendance tbh is a no go for me, everyone is using a speed booster in my guild and I get very behind on everyone, so it makes my heals useless. I try to wolf up, go fast put up a HTT on second run because most people prob would've used their cd on the first one. I use Asendance/Spirit Link on first and second try separately. Resto Shamans (atleast for me) is pretty weak untill you get to the boss and can aoe like mad.
The more important question would be: why are they taking so much damage in the first place? You can get to the other side on normal without receiving a single heal. What I find easiest is to just get to the other side in ghost wolf and just heal the approaching late stranglers.
(obviously different for hc)
Riptide everyone. Its not that costly and its a good hot. Healing tide the second one. As if you do it on the first one, it'll end up having more overhealing (less intensive).
Higs is correct. on normal, assuming people do not take avoidable dmg then they can make it to the other end without heals. they can always pop a healthstone or personal CD if needed. just get to the other end yourself and heal once you get there. you can always drop an HST the second that you are dropped off to start the phase as it will fire a few heals off while folks are still in range, RT yourself and get moving in GW form.
How many heals are you using? 10 or 25man? how many stacks does the boss have going into the 20% phase? Just not alot of info here to really go on, but what I found for our 10man group was that 2healing was a better idea than 3, as he had alot less stacks going into that phase, enough of a reduction that we could survive until we all stacked up, then it was a simple matter of healing up before the 10% switch.