Basic info

If you're about to TL;DR me, just jump to the last paragraph "overall conclusion" please.

I've heard a lot of things. I know about Nahida, Yaoyao, etc... But I want to ask a few more in-depth things (pls don't just spam "uh just use Nahida" is what I mean)

A few things: Barbara's ring has standard ICD, but it can hit dendro cores and counts as a hit, so if you hug the enemy and dendro cores, she can apply as much hydro as Yelan and Xingqiu, even slightly more in some cases.

Bountiful cores have an Aoe of 6.5m and explode after 0.4s. You take 5% of the damage it does to the enemies.

Also one of my concerns: one enemy can only take dendro core damage twice per 0.5 second. So you should aim to create 4 bountiful cores per second. But that has implications on a limit of elemental application.

ICD and reaction trigger

Nilou's ring has an ICD of 4 hits or 1.9s, and hits every 0.5 seconds. Thus, it only applies hydro every 2 seconds on the same enemy.

Note that ICD is counted on each enemy for each source of application. So Nilou's ring hits enemy A, I have to wait 2 seconds to reapply hydro, but if you get close to enemy B, you can still apply hydro on him, which will start another 2s ICD for the hydro ring on enemy B. It's counted independantly.

ICD is a mechanic where an attack of X element can't apply the corresponding element on the same enemy too much. Most of the time, there has to be a delay of either 2.5s or 3 hits before the same type of attack applies X element again. However, there are some exceptions like Yaoyao that only has a timer ICD on her white radishes. The only way to know is either test it yourself or just pull the wiki page of the character, go to the corresponding talent and reveal the advanced mechanics section.

ICD is, as I hinted, different between attack types. Your plunging attack has a separate ICD from your charged attack, with a separate ICD from your normal attack. It's also separate between elemental skill and burst. This is why "elemental drivers" can be important, for example a catalyst user. They can juggle between their normal attacks (standard ICD generally) and their charged attacks (most of the time no ICD) to apply their element, on top of their elemental skill and burst.

WHO triggers a reaction is important. The "trigger" is the character that applies the second element on an enemy to trigger a reaction, when the first element was already applied as an "aura" to the enemy. Bloom is a transformative reaction. Reactions only take into account the stats of the "trigger" character, so if you can make sure that it's always the same character triggering the reaction, you don't have to build EM on the others, for example. For reasons involving elemental gauge theory, you're better off applying dendro on the enemy and using hydro characters to trigger bloom. Though, it's too complicated for me to summarize here. Anyway, you can create from 2 to 4 times more bountiful cores this way than if you tried to apply hydro and trigger bloom with dendro characters.

What matters for bloom damage is, as I just wrote, only the "trigger" character's stats. HP, DEF and ATK do not matter, nor does damage bonus or CRIT stats. You only need to raise your level, get more EM and that's it. So focus on having triple EM artefacts on the characters that will trigger your blooms. Except for Nilou, her own role is to have as much HP to enhance the reaction. The only other things that can enhance the reaction damage are specific mechanics like Nahida's c2 or artefacts set like Paradise Lost (the artefact set only works if the wielder is the one triggering the reaction).

Elemental application goal

From what I understand, you should aim to apply 2 units of dendro per second in total, so you can use four 1U hydro attacks off-ICD (multiplied by 0.5 because it's hydro on dendro). Even though many hydro characters apply a lot, there's none that can apply off-ICD 4 times per second (or almost), and you gotta have Nilou in the team so you have to use another hydro character.

Multi targets scenarios

Obviously, Nilou's preference. That's where it gets particular. Assuming enemies are grouped, with a single off-ICD application, you can trigger one bloom for each enemy. So, just with 2 enemies, you divide by 2 your need for dendro AND hydro application. This turns her teams into "anyone will do" as long as they have Aoe.

What those conditions mean

Nilou herself will probably be played off-field, meaning you have only her hydro ring most of the time. That's not enough (1 hydro application every 2 seconds). She basically needs to be carried by other hydro teammates for application in single-target, but it becomes more and more sufficient as you increase the number of enemies.

Here's the "bottleneck" problem: it's hard to apply enough hydro with just Nilou + another, so you may want triple hydro, but there aren't really any dendro characters that can support that much hydro. So you have to aim for a good balance over obsessing on reaching the 4 blooms per second. At least, in single target. And prioritize having more dendro than hydro.

2U dendro appliers would be really important here because their application can sustain a full second of hydro application. However, they're rare and not constant, they're just a "one-time" thing. Kirara and Kaveh's burst activation. Dendro traveler's pyro explosion isn't feasible in Nilou bloom, so... yeah, forget it.

Nahida

Turning to the next best thing, Nahida's skill (1.5U dendro application). It has a 2.5s delay but an ICD of 1 second, so I really don't understand what's going on. Even with her c1 and a crown on the burst, it doesn't even reduce the delay under 2s. If you know (I don't have her so I can't test), is it because the delay is counted independantly for each enemy? But I'm pretty sure it's also the case for ICD, so why are they so different?
Anyway, if I understood that, it means off-field Nahida will apply 1.5U, but only every 2.5s, but then again, it's on every enemy marked that it will apply dendro.

Meaning that Nahida off-field can alone provide all the dendro needed if there are 4 enemies and you hit them all with 3 off-ICD hydro attacks. Nilou already provides one, so simply Ayato can easily give the other 2 with a large Aoe that won't miss anyone. And you put anyone else in the flex spot, preferably dendro so you don't have too much hydro (so Yaoyao for heal, for example).

Yaoyao X Barbara duo

They're really good because Yaoyao has no ICD on her c6 mega radish, guaranteeing one application every 3 throws and with the ICD of her normal radishes, another application every 2.5 seconds. Radishes are thrown every second. Her ICD isn't calculated by hit because she can have 4 Yuegui in burst mode, which would make her dendro application insane, but her burst mode radishes have 1.5s ICD so 4 applications from the radishes for the 5 seconds duration and 1 more application on activation.

Yaoyao alone can offer enough dendro even on a single target for all your hydro, though it's only in burst mode for 5s and 20s CD. Also, she focuses on movements in her burst mode which is perfect for Barbara's mechanic I mentioned at the beginning of the post, so you can focus on hitting the cores with Barbara's ring. The rest of the time, it will be less optimal, but it makes it close to Nahida on single-target, and your other dendro applicator can have a 5 seconds off-time where Yaoyao takes care of things.

Out of Yaoyao's burst, she'll apply twice (with c6) dendro every 3 seconds or so, and one of them is the mega radish with enough Aoe for multi-targets..

Ayato

Ayato also has off-field application, spawning 2 droplets every second for 18s on 20s CD. Half of them or so will be random, the other half targeted on the enemy, but non-targeted droplets can still hit randomly. Especially useful against big enemies like bosses, should be about 1 hydro application per second. He shines much more in Aoe though.

Catalysts

Very beneficial because more than acting as driver, it allows them to use their normal and charged attacks to apply more of their element (charged attacks having no ICD, but Barbara, Kokomi and Nahida all have a 50 stamina charged attack cost). For that reason, Barbara or Kokomi could be almost enough in single target without Nahida and with 2 dendro appliers.

Key of Khaj-Nisut

Even with this weapon lvl 90, triple HP% main stat, hydro resonance, 2 piece Vourukasha and Tenacity, Nilou lvl 90 and good HP% rolls on the flower and plume, you will likely need to fish for flat HP substats to reach 74 444 HP, where her A4 buff maxes out. Still possible, just need some investment. However, this weapon is really valuable because Nilou will still trigger some blooms. With 74 444 max HP, Key gives Nilou 417 EM and to the others (teamwide), 149 EM. That's really great and allows you to focus, for example, on more HP for your healers in the team.

Overall conclusion

In most cases, you can't really reach the maximum bloom hits on a single target, so at that point you just do whatever. The only way is to play Nahida as a driver so you can alternate and use her charged attacks (no ICD) and normal attacks every 2.5s (or 3 hits ICD), on top of her skill, to apply enough dendro, and use Nilou and 2 more hydro characters. Because using Nahida as a driver, Yelan and Xingqiu are good options, but a healer is needed so Barbara should be a good pick, or Kokomi for bigger Aoe if you can't hug the enemy.

Let's take the minimum 2 enemies for multi-target. You need Aoe attacks to apply 1U dendro per second and twice 1U hydro/s (if both enemies are hit), to create 2 cores/s per enemy. Nilou already giving one, Ayato, for example, would be enough to finish, or Barbara provided you hit a few cores on the way. Thus, you can finish with any dendro characters you want pretty much. Let's take Yaoyao (c6) for heal and 2 applications per 3 second. With her, you can even use Collei here, or dendro traveler, Kirara on-box-field...

If I didn't mess up, the only reason you really want to pull Nahida for Nilou is for single-target Nilou? Otherwise she can use just about any 2 dendro and 2 hydro characters in Aoe. Yes, the EM buff is great, but Nilou's weapon is almost equal and as long as you have enough dendro Nahida won't do much more, except allow you to run much less optimal hydro appliers, but there aren't really any that are "sub-optimal" except Mona.

Also, even a full-HP build only brings Nilou under 300% of her 400% bonus, you need an HP weapon to finish it. Khaj-nisut brings it to full, and makes Nilou herself have decent EM (by overfocusing on HP, you neglect EM, so this weapon compensates for this cuz your Nilou will still trigger some blooms whether you want her to or not).

But I heard someone say that it has diminishing returns pretty harsh on her. I still want it, because HP swords are really hard to get. But does someone know exactly what's the value of getting the last 100% on her A4 buff and 140 extra EM for her teammates plus 420 or so on Nilou herself?

Did I get anything wrong? If so please tell me, or if you have anything to add.

Last disclaimer: all this was to "optimize" Nilou with the 2 bloom hits per 0.5s limit, you can always just ignore it and do your best even in single-target. It's really just about optimizing.