Why Runeslayer?
Runeslayer allows the Opus to be mainhanded. Pair this with the Opus 3rd skill of Falsehood (Echo), the ougi now gives all allies Double Strike.
If you only have 1 opus and it is using the Chain of Falsehood, it can make playing other content miserable as it reduces charge bar gain by 100%.
A solution is by grabbing another Dark Opus (the primal one) and make the other one use Chain of Falsehood, while your main opus uses a more general skill such as Stamina.
Couldn’t it be any other Katana MC? Runeslayer comes out ahead because of the 1st Skill, Runeweaving.
Runeweaving
This gives MC instant full bar. So it allows you to instantly Ougi and make use of the Double Strike.
Multiple Ougis
Pair this with Splitting Spirit (Obtained by unlocking Mystic Class), you can instantly Ougi 2x in 2 turns. Allowing the usage of 2 turns of Double Strike.
We can further extend this by using Qilin as friend summon. Qilin call resets all recastable skill cooldown. Effectively giving us 4 turns of Double Strike.
Sample Grid
Opus 3rd key MUST BE Falsehood (Echo). Seraph can be replaced with Ultima if it's ULB for On Element fights.
For off element fights, you can replace the seraph with anything, most likely more damage mods as you probably would not reach the cap.
If you use skill nuking characters like Bowman and Seox, you can slot in Agonize / Celeste Saber Ater for more skill DMG.