This is a build focusing purely on [Koumei]'s status afflicting abilities, ignoring her decrees. This gives her very competitive up front damage with adequate enough range on her 4 to nuke.
You can always switch Sickening Pulse to replace her 1 instead of her 2, but her 2 is rather awkward to play around when you aren't doing a long endurance mission.
The forma on the R0 [Archon Continuity] slot is there in case you want more duration.
Important: In order to make use of the max corrosive stack shards I recommend using this hound primer build: https://overframe.gg/build/812377/

Arcane Impetus

Makes [Koumei]'s abilities even more spammable, as it is extremely easy to proc with her 1st and 4th ability and gives her just over the equivalent of [Streamline] and almost as much strength as [Blind Rage]. Sickening Pulse will also refresh the timer, guaranteeing that you get the maximum bonus.
*Since Tau damage cannot proc from anyone except [Caliban], the bonus is limited to 13 stacks, which is realistically the highest you can expect from this arcane.

Arcane Energize

More energy = more ability spam.

[Archon Continuity]

If you want more duration simply change the rank 0 [Archon Continuity] for a higher rank one; duration isn't all that important on [Koumei] and means that sickening pulse can be cast repeatedly faster. The downside of low ability duration is that you will be casting your 1 more frequently, but in my opinion a ~5s increase isn't enough to justify putting a max rank mod.

[Archon Vitality]

This mod is purely here for its secondary heat effect. I had Archon Flow here but the benefit was minimal considering how fast you offload energy w/ [Koumei].

Archon Shards

1 tauforged casting speed (amber)
1 tauforged [Equilibrium] (violet)
1 tauforged toxin status effect dmg OR 1 tauforged ability damage on corrosive (emerald)
2 max corrosive stacks (emerald)

The Exilus slot is flexible, but [Speed Drift], [Enemy Sense], or [Primed Sure Footed] are probably the most relevant.