That boss is hard to heal because you need to be closer then 25m to the boss to be able to heal the tank.
So I discovered. Before pulling, I had the tank indicate where he would be standing and then moved to the extreme range of my healing, in theory 30m, but this is still apparently within range of the boss stun - I guess because it's being tanked against the wall so the boss is actually a little closer than the tank.
One thing I noticed that helps me with healing is that just before the aoe stun there is a debuff on the tank, looks like a fist hitting the ground, you have a second or so to run away and get out of 25m range of the aoe stun. You can run back even before the aoe stan damage is finished and continue healing the tank.
Now that sounds like it could make the difference, I've found that the constant interruptions in healing gradually take their toll and grind the party down - so thanks for the advice.