Introducing: The True Dual-Form [Equinox]

Before anything, the build absolutely require shards.
The goal is 140%str, to full strip with a simple cast of terrify, so it's missing 45%str, either 3 tau-forged rad, or 5 red.
Provoque can probably help in reducing the amount for shard needed, but I don't know the threshold values.

Day form for nuking efficiently

[Maim] + armor strip = big dopamine [Rush]. That's about it really.
But it's the squishy form, so [Rolling Guard] is there, and a weapon that can properly delete small amounts of mobs is kinda necessary (I use a [Quellor] (build available on my profile))

We also have an augur mod to restore a tiny bit of shield.
And Arcane Intention that more than double the hp of the frame.
Both help to at least reduce the chance of one-shots.

Night form for chilling with weapons

Built in DR + auto shield recharge = chill as long as you kill.
That's the tanky form. With 140%str, Pacify reduce enemy dmg to a third of their nominal value, and approximately cut their speed in half.
recommended: a focus str buff ability (I use Sling Strength from the Madurai tree, which brings me to about x0.28 dmg and 72% slow)

Considering buffed values, ehp are multiplied by about 8 (pacify has a strange relationship to range, so it's not a hard value).
Once again, Arcane Intention helps the ehp by doubling them, but more than that it has an interesting interaction:

1. Intention function like a heal on recasts. It gives hp and max hp when activated, and take back only max health on deactivation, leaving the player with a net positive.
2. Since it take back max health on deactivation, it means it also trigger [Peaceful Provocation]'s night aspect, meaning you can scale the slow effect just by recasting mend! (6 or 7% per recast, but still, no need to take dmg)

Questions:

Are Shard really necessary?

Truth is, [Creeping Terrify] doesn't really do much for the build, assuming it's only ever directly followed by a map [Swipe]. You could replace it with [Umbral Intensify], in fact that's what I had on my previous build.

Are Arcanes really necessary?

Intention you can play without I guess, tho Nigh form would be much more squishy.
But energize is pretty much necessary yeah. Sorry.

Why not use Arcane Blessing instead?

Blessing is a solid alternative, but it needs ramping up, and as such it's not great on shorter missions. Plus the bonus just vanishes if you die. And everybody and their mother use it. Long story short, it's a preference thing.

How much [Maim] should I have before I release it?

Let's do some math:
This build's [Maim] has 42m or range. At the epicenter the dmg is nominal, and at the edge it's null.
Let's say there's a mob at 30m of you. According to the formula given by the wiki, it will receive 28% of the accumulated dmg. But it's a pure slash aoe on stripped grineers, so they take 1.25x the dmg, for a final value of 35% of the displayed number. You'll need to accumulate about 3 times the mob hp to kill it.

[Maim] accumulate 75% of the dmg dealt to mobs in it's radius, meaning to accumulate 3 health bar, you'll need to kill exactly 4 mobs.
4 mobs. And the whole map dies.
But realistically, some units have more health than others. For exemple, eximus. Kill one eximus, and you basically have a guaranteed map [Swipe].

But eximus also need to die. From experience, killing 3 eximus guarantees eximus nuking in a 13m radius.
But hey there's no easy way to calculate this stuff on the fly, you just got to get a feel of when your Maim is ripe, depending on what you fight.

I can't charge my maim, I die too fast/kill to slow!

Yeah it's proper squishy.
Since [Maim] doesn't need much to get [Swipe], you can cast terrify before releasing your [Maim]. As the mobs flee, it should help get pressure off of you too.
And since it only needs a few kills, I'd recommend guns that can kill very quickly isolated targets instead of big boom weapons.
I already said I use the [Quellor], but I can imagine the [Phantasma] working well, or perhaps some of the new incarnons. Big shotguns to the face might work too.

I run out of energy in Night form !

It's probably because pacify & provoke.
Pacify & provoke works like Gloom, or rather the other way around. But Gloom as a drain cap, Pacify & provoke doesn't (#thxDE).

Well as they say, offense is the best defence. If you have too much mobs on the terrain sucking your energy, it means there's too much mobs on the terrain, and a big aoe should remedy that.
And don't worry about wasting augment charge. It's serving you, not yo userving it. And there'll always be a random ally of a pet to lose health in your place anyway.