For mythic+ the ideal hero tree is Chronowarden, it doesn't add a new spell and instead focuses on enhancing and improving every part of the base kit of the spec.

Where does your healing come from

Your healing sources in M+ are very varied, and different spells can edge out others depending on how you play. It can be normal to see any spell as your highest healing source in your breakdown depending on situation, group composition and playstyle. As a general rule, your main answers to damage should be sb, ta, ve, rev via gh, and db.

sb is a short-ish cooldown aoe burst heal that will smartly pick the three players with the least amount of hp and heal them alongside your target. Most of the time you want to cast sb on yourself for two reasons: First because of how our mastery works, casting sb on yourself means you get healed by it a couple of milliseconds before everyone else, which means your mastery will be active when healing the other targets. And second due to lb a lot of the time you will be transferring healing done to yourself to everyone else.

db is a frontal cone heal that has two components, one part of it it's done instantly at the moment you cast and the rest is done over time. The higher you empower it the stronger the initial heal gets at the cost of a shorter (and thus weaker) heal over time. Most of the time you will cast db at rank 1 due to it being your best tool to respond to rot damage, while upranking it is usually not needed because sb and ve cover that role better. In the scenarios where you don't have other options available you should uprank it as needed.

ve is a very versatile spell. If buffs your db by giving you coy, it can increase sb healing via lb and it also does very strong healing on it's own. Using ve correctly is one of the hardest parts of mastering Preservation, as juggling all the different uses of the spell appropriately while also leveraging it as a movement tool without wasting part of it or killing yourself in the process is a complicated thing.

ta is a set up and complement spell, it helps prevent damage by applying big shields on everyone every time you cast it and also applies echoes via reso to make your follow up healing even stronger. You should constantly use ta very often as it is a great way to always have extra protection on the group and make it easier for yourself to respond to unexpected events.

fb is also a very important healing tool for dungeons, due to lgf casting fb on aoe will also heal your whole party for a large amount, and as a follow up you gain leaping which can be used either for extra damage or as an extra aoe healing tool. You should always cast fb at max rank.

Finally rev is a complex tool that can vary from very good to useless depending on specific situatiosn. gh is incredibly good at responding to sudden burst damage on a single target, as echoing the rev will cause two ghs on the same target. You can also apply rev to everyone via reso to increase your healing on the whole group with gp and generate procs of lifespark to get instant chronof for healing or damage.

Important things to know

• ta is an incredibly important part of your kit. The shields it applies are very strong and shields in general are an incredibly strong form of healing, on top of that the extra echoes it applies help you jump start your ramps without having to use a ton of extra global cooldowns applying echoes manually. However this does mean that you should ignore your manual echoes, remember that manually applied echoes are more than twice as strong as the ones ta puts up. Effects like gh and lb scale to the strength of the echo so using them on echoes from ta makes them much weaker, while a lot of the time this won't be an issue in the specific scenarios where you need high amounts of healing you will need to use manual echoes. Finding the correct balance on which echo to use in which scenario depending on your available resources and how much healing you need is the key to mastering Preservation.
• Both sb and db will do extra up-front healing thanks to afterimg. You don't have to actively do anything about it as it will happen on it's own but you should remember that both empowers will also heal three (five during tempburst thanks to s3chrono4p) targets with chronof and this can also lead to some bursts being generated.
• lb doesn't care about range, if you find yourself in a situation where you need to heal people far away, you can echo them preemptively and then ve when the damage hits, followed by healing yourself to transfer healing to them as needed while they are out of range. ec is particularly good for this.
• tts is one of your strongest tools and you should focus on using it properly as much as possible. Casting an instant max rank sb is a very strong way to recover from an emergency, and on top of this you gain tempburst for the next 30 seconds making whatever followup you need to do much more smooth. Thanks to s3chrono4p your chronofs will cleave two extra targets during tempburst and this acts similar to leaping in regards to targeting: If you are fighting a single target boss your damage chronofs will send the two extras towards nearby injured allies for healing. This is also on a one minute cooldown due to s3chrono2p.
• zephyr, caut and tg are very important tools for M+ that you shouldn't forget about.

Gameplay

The Preservation kit is very versatile and you can use almost all the tools for whatever type of healing you need if you adapt, but as a general rule you can consider these the most appropiate responses to different emergency situations:

• To heal aoe burst damage: You main tool will be sb. You can use it directly on the group, echoed either manually or via ta depending on how much healing you need; or you can echo a ve on the whole group to heal them and apply lb followed by the sb to transfer via lb (still at max rank). Another alternative is to use fb on aoe for lgf healing or using leaping to heal the group.
• To heal aoe rot damage: You can cover the group in rev and db, and increase either of them via echo and ta as needed. You can also manually echo everyone and let the damage happen until they hit low hp and then bring them back up to full in a single hit with sb.
• To spot heal single targets: Your best option is to echo and rev the target, as gh will heal them for more than 30% of the total hit plus add two copies of rev on them and increasing all the healing they take by 21% thanks to gp. You can also use any lifespark procs you current have on them.
• Constant damage on a single target: You should cast all of rev, ve, db and sb on that single target and increase some (or all) of these with manual echoes as essence allows. You can also rescue them to apply tg. Spamming hard casted chronofs on someone should be your very last, desperate resort. If you find yourself doing this very often it most likely means you are missplaying

You should use your empowers often and frequently but also being careful to not overcommit short cooldowns making you run out of healing. Pace yourself and only use resources as needed. Both sb and db keep healing over time after the initial application so you have to notice when it's ok to not cast anything else and just let your heal over time do the work.

ta is used as a quick way to apply echo to the whole group to make your follow up healing stronger, but manually casting echo on the group results in a bigger increase. You will choose between one or the other depending on how much healing the specific situation you find yourself in requires and how much time do you have to prepare echoes for it. The only important consideration for choosing ta echoes as opposed to manually applied ones is that gp does not scale to echo strength and is full power regardless of how you apply it.

Conclusions

Chronowarden Preservation is an incredibly strong and versatile healer that suffers from high complexity and requires very fast and constant on the fly decision making. You have a lot of tools to do very high healing but all of them are limited via resources or cooldowns so you need to properly decide how to spend them and pace yourself properly. It is possible to "run out" of healing when playing the spec, which means you simply don't have any more tools to respond to damage and you just can't heal, which is a very unique and annoying problem for newer players that gets solved when you start learning how to better sequence your spells. When played properly the amount of healing, damage, and even utility that Preservation can bring is unmatched, but playing it properly is incredibly hard to do.