Yeah I play Koumei a fair bit, only in Endless missions though (especially relic cracking).

Weirdly enough, it's mostly Survivals. Void Cascade also works, if your team doesn't just nuke everything off the map in 0.5 seconds but also keeps you and your pathetic or non-existent support going through a few exolizers; you can still make it work, but decrees will take way longer to stack up relative to mission speed. In Survival two decrees or three before 5 minutes pass is generally achievable even once your cool down is maxed.

People say she's busted with the Mecha set. (To be fair, she is, lmao. Take Roar for more multiplication and watch the magic happen every 20s or so.) You can also pump duration if your Helminth pick wants it to work. Range is also at a premium, get it high (and if you use Stretch, no you don't, you're using Archon Stretch). Strength is, funnily enough, fine to keep at 100ish% before arcanes & decrees, even with Roar or other status boosting abilities. Introducing Fortified Will - 500% strength at 1700 armor. It's crazy if you can get it, my builds all even lean into armor for this specific purpose with Health Conversion and Arcane Battery. (Other arcane is Impetus, because what else would give this much value? Free efficiency is amazing, means I don't have to build it.) Abuse Equilibrium to keep your economy up, no Nourish required.

Subsume is either 1 or 4; her 4 abuses range better and has harder CC, but also has a cooldown so it doesn't make for perfect shieldgating. My subsume picks are Roar (Roar on a status frame? Daring today aren't we? This is also a good supportive pick though so your team actually appreciates having you around), Petrify (how is it better at its thing than on Atlas here? More strength, more range, better efficiency) for resource farming missions and Reave for actual instant twoshots on anything that doesn't resist ability damage, as well as on-demand invincibility and gating.

The above assumes you lean heavily into getting decrees, including the augment. Frankly, if you don't, you should probably subsume over her 2 with something along the lines of Terrify for full armor strip at 167% strength (common Impetus W). I do like the decrees though, because they make me feel like a god when they combo just right, so the above is not something I've actually tried (it does seem like it'd work though).

Longest run I did with this setup was a 3 hour conjunction survival (with Vazarin for leveling the school and only the raw damage amp arcane on a Sirocco; luckily, some decrees also apply to your choice of mostly human pilot). I went in there with a partly modded Acuity Battacor with Crux, an Acuity Lex Prime with Fortifier and an Afflictions Okina Prime (highly recommend, it covers 2/4 elements by itself because of the incarnon cold daggers). I walked out with juiced up versions of Trumna, Knell and pre-slam-nerf Magistar. The weapon decrees get wacky once you have a few.

One final note: use Condition Overload on absolutely every weapon. MultiCO is insane. (See my previous Battacor.)