Raids

In raids your best hero tree is Farseer. Mana management is an important part of your gameplay and pacing your Chain Heals properly so you can keep healing for the whole duration of the fight is something you should actively think about. Raids let you prioritize efficiency more and you can plan your gameplay around what things will do the most overall healing across the whole encounter.

Where Does Your Healing Come From

Your most important sources of healing will be Healing Wave, Chain Heal, and Stormstream Totem. This doesn't mean you should ignore other parts of your kit as several things are heavily interconnected to benefit each other. Riptide is a key spell because it can let you cast more Stormstream Totems or activate Ascendance which empowers your filler spells, it also gives you more stacks of Undercurrent and more targets for Deluge so even if its not a major source of healing by itself using it directly leads to improving upon the spells that will do the majority of your healing. Similarly Unleash Life won't do a lot by itself but the healing increase it provides to your other spells is significant. You can consider these kind of like "set up" spells, they are important because they improve your other abilities that will noticeably move health bars when used.

With a significant portion of your healing coming from spells with no cooldown that you cast repeatedly also comes a very important thing to keep in mind: You actually have to cast them as much as possible. Your short cooldowns are good and very versatile but you need to spend all the free time you have after using them casting your Healing Waves and Chain Heals as much as possible in the correct targets. Downtime is inevitable up to some point but if you find yourself not casting anything for large periods of time when there is damage to heal you are doing something very wrong and should plan your Spiritwalkers Grace better so you can keep casting.

Restoration Shaman doesn't ramp nor does it have long sequences of preparation before doing heavy burst healing, it simply presses spells as needed and healing happens. You don't need to force yourself to always be casting during the whole fight but if there is people that need healing and you aren't casting heals you should have a very good reason why.

Important Things to Know

The vast majority of your Stormstream Totems will come from pressing Ancestral Swiftness. The random procs from Riptide are good but overall rng does not play a significant factor on Stormstream healing.
Healing Rain is good, both from the healing it does and from having more targets that benefit form Deluge. Aim to keep it active as much as possible in a clump of players that you know won't move out of it.
Healing Stream Totem is not amazing but its still good enough to justify casting it. Aim to use it when you have to move or a bit before damage comes in so it helps you heal during it but doesn't take away globals during dangerous moments.
Both Healing Wave and Chain Heal are good but serve different purposes. Don't stick too hard to either one and use the correct tool for the situation. This is different during Ascendance however as Healing Wave is simply a much better spell for the duration of it.

Gameplay

Your gameplay will consist of using all your short cooldowns on cooldown as much as possible and then filling in the extra time with one of Healing Wave or Chain Heal. Your main goal is to minimize downtime and simply cast your spells every time they are available. The only proc you need to react to is Deeply Rooted Elements and that just means you really want to cast a lot of Healing Wave for the next 6 seconds, besides that you can smoothly plan your casts ahead of time but seeing what will come off cooldown first

Use all your Stormstream Totems, most of them will come from Ancestral Swiftness but you can also get one every time you press Riptide. The buff lasts for a decently long time so you are not in a huge rush to use them and can bank up to two of them but you also don't want to hold it for too long. If there is damage to heal and you have a Stormstream you should use it.
Cast Riptide as much as possible and on different targets. Spreading Riptide gives you more stacks of Undercurrent and more targets for Deluge. It can also proc Deeply Rooted Elements and Stormstream Totem so there is a lot of very good things attached to Riptide.
Use Ancestral Swiftness on cooldown. The most important part of this is not losing Stormstream Totem procs but it also spawns an ancestor and gives you a mana-free cast which is very good. It doesn't mater a whole lot what you use the free cast on but Chain Heal is generally a better choice for it.
Use Unleash Life, the healing by itself is very decent and the follow up buff to other spells is very good. Your tier set improves the spell a lot and the bonus plus the ancestor it spawns makes it important to use the spell every time its available.
Keep Healing Rain active, on some fights you will want to drop it on the melee near the boss and others the ranged stack is a better place, it heavily depends on how the boss fight plays out. The healing it does is good and having Deluge active on many more players is very very good.
Use Healing Stream Totem as a bit of an instant filler spell when you have to move or you know damage is about to come in in the near future. The spell isn't amazing but its still good enough to be worth casting but ideally you want to use it so it is active during heavy damage periods but it doesn't take away important global cooldowns during them. It can be fine to drop both charges at the same time when you are a few seconds away from a big wave of damage hitting.
The rest of the time heal people with either Healing Wave or Chain Heal. Chain Heal does a little bit more overall healing but costs more mana and it requires that your chain jumps properly on all the targets with only a 20yd jump range, if the ideal conditions are present then this is your highest hps choice but a lot of the time you will prefer Healing Wave because you would rather save the mana, because the damage is focused on a single or two targets rather than spread across the group, or because you think the group is too spread apart and are worried Chain Heal might miss jumps.

During Ascendance both Healing Wave and Chain Heal improve massively, however Healing Wave gets very far ahead because on top of doing a very high amount of healing it also restores you more mana than what it costs to cast due to both the main hit and the half-effectiveness cleave both proccing [Resurgence] every time as they are guaranteed critical strikes. Simply pressing Healing Wave as much as possible during Ascendance results in high throughput for zero mana cost which is incredibly strong. To maximize your healing going into Ascendance you want to follow a bit of a sequence to buff yourself:

Use all your charges of Riptide to get stacks of Tidal Waves and to not waste the cooldown.
Press Unleash Life to spawn an ancestor and get the healing buff from it.
Use Ascendance.
Cast one Healing Wave, this cast will consume your Unleash Life buff to do more healing and cast faster
Use Ancestral Swiftness. If we use it before we would waste the cast time reduction from Unleash Life as both buffs would be consumed at the same time, by using Swiftness only after Unleash is gone we maximize the cast time reductions.
Cast Healing Wave repeatedly until Ascendance is over.

During Deeply Rooted Elements simply try to cast as many Healing Waves as possible. Do note that you are not casting Healing Wave during Ascendance "just to restore mana", the spell does more healing than anything else you could be casting during it and the mana is just an extra bonus on top.

Conclusions

Farseer presents an incredibly strong option because of how well it interacts with Ascendance and its ability to proc Stormstream Totem way more often than Totemic, these combined with the better healing profile offered by having most of your healing come from abilities that you directly target on the players that most need healing results in a very fluid gameplay that feels very impactful.

The gameplay of managing your mana by restoring a good portion of it during Ascendance and then dumping it with Chain Heal outside of it is a very interesting design that lets you play with your resources and plan ahead depending on what the situation is gonna need. Overall the spec is very simple but it is very fun to play and offers some interesting decisions about what you want to do and what would the best way to do it be.