Not a healer here but someone who has done a lot of vSS hm runs. Check if your dds have cp into increased healing received, check how much max Health they have, (I used to run into ice on a magplar with roughly 21k hp needlessly to say that going on a stamcro with around 18k is easier to heal), if possible depending on class let dds use some sort of hot on themselves Right before entering the ice (vigor for example if stam dd).

Thanks for tips guys. Kuratius I dont understand this gcd term you are using. Isnt orb a nice healing abilitysince it has more hps in tooltip even thatn illustrious healing?I couldnt use sanctuary on body pieces because I had jorvuld there. Isnt jorvuld just better because it increases duration of major vitality buff from vBRP staff?

I was actually using healing ward and illustrious healing. Just failed to type the proper morph names in my message.

My recommended setup (as a solid starting point) for this role as a Templar are:

Sets: Olorime, Jorvuld, Troll King, BRP Resto (Double bar Resto, with Olorime on one bar only)

Skills:
BRP Bar: Symbiosis, Radiating Regeneration, Healing Ward, Breath of Life, Power of the Light
Olo Bar: Channeled Focus, Combat Prayer, Extended Ritual, Illustrious Healing, Energy Orb

Healing "rotation" is as follows: Make sure Symbiosis is toggled on before you begin healing the cages. Pre-buff the ice cage with 1 cast of Illustrious Healing, before the person enters the cage. Immediately cast Healing Ward as soon as they enter the cage-- it is important that you make sure the caged person receives the Ward so they get Major Vitality + the HoT from the skill itself, so keep your eye on the person's health bar and watch for the shielded effect. Then, light attack weave using Symbiosis + Breath of Life until the person is full health.

Make sure that your DDs do not enter both cages at the same time. The 2nd cage person should wait until the 1st cage is healed to full, and then enter, in order to give you enough time.

The reason I recommend double Resto is: You will benefit from two important Resto Staff passives this way, no matter which bar you are on, those passives being "Restoration Expert", which increases your healing by 15% on allies under 30% health, and "Restoration Master", which increases healing with Resto staff spells by 5%. As the healing debuff from the cages is quite intense, these passives will help you tremendously.

I do not recommend Sanctuary, even to those just starting out, because its range is quite small (just 10 meters), meaning you have to be quite close to the caged person to see the benefit from this set. Often times, you will find that the 2nd cage spawns far from the first, and you should not wait until you are within Sanctuary range to begin healing them; that being the case, you will find that this set bonus is wasted for those moments where you are far from the caged person. You could argue that you can begin moving toward the 2nd cage as you finish healing the 1st to be in range of the 2nd when that person enters, but then the same issue arises in reverse in that you move from Sanctuary range when you finish trying to heal the 1st. All in all, I don't think this set suits the dynamic and mobile nature of this role, and furthermore I think that the 5 piece bonus is wasted for the rest of the fight, as the extra bit of healing is not at all needed.

I think Jorvuld is a wonderful set to use, one, because it increases the duration of Major Vitality from the BRP staff, and two, because it increases the duration of Minor Berserk and Major Courage (and Major Force, if you are using Horn). So with Jorvuld, you get many benefits-- increased healing strength for the cages and longer-length offensive buffs, whereas with Sanctuary, you just get stronger heals, so long as you are in a specific range.

Troll King is also a good set to use to help heal people from the cages as quickly as possible as it is guaranteed to proc on who you want it to proc on, every time. With Bogdan, though, it may or may not proc where you want it to, and even if it does happen to proc on the 1st cage, its cooldown will make it so that it never procs on the 2nd.

You should also use Illustrious Healing rather than Healing Springs as it is a stronger heal by far, and every little bit is important to combat the huge healing debuff.

As far as sustain is concerned, you will find that the faster you heal people from the cages, the less you have to spam heals on them, and the better your sustain will be. That is why I really recommend that you try the setups that I suggest, at least when first starting out.

That being said, one mistake that I have seen people make in this role is using 3x Spell Damage glyphs, their goal being just what I said, to free people from the cages as quickly as possible by means of super strong heals buffed by their Spell Damage; however, you will find that this setup is more harmful than helpful as you will likely run out of Magicka before the 2nd set of cages because your regen is too low. You need to find the proper balance of having enough sustain to heal both cages quickly, and enough power to do so. Maybe this is a bit different for everyone, and also based on how long your total fight time is-- the more cages you have to heal, the better you will need to manage your resources, making sure you have enough sustain to get through them while supporting your group.

Experiment and find a setup that works best for you. For me, on Warden, I am comfortable using the Apprentice Mundus, 2x Arcane Mag Recovery jewelry, and 1x Infused Mag Recovery. Maybe you might benefit more from using the Atronach Mundus, though. You could also try 3x Arcane Mag Rec, or use some Spell Damage glyphs if you need. In my opinion, there is no right answer for this as it is something that could very well be a bit different for everyone, based on their own level of comfort, and their group's experience.

Finally, I'd like to share my video of my performing this role. Though it is on a Warden, the concept is the same, and you can see how I add my own personal twist to it.

My gear is: Olorime, Infallible Aether, Sentinel of Rkugamz, and the BRP Resto. I am using the Apprentice, 1x Infused Mag Rec, and 2x Arcane Mag Rec jewelry.

You can see that you can do the role using a number of different sets (I've even seen some groups where the cage healer uses Ebon, lol), but I think the first setup I recommended to you is the strongest in terms of healing power while offering great offensive boosts for your DDs.

https://youtu.be/VANSjwqmPHE