If your Sigret is already doing Wyvern, I saw some posts recently about a one shot build and you only need the units to be built in order per speed. Not a high gear requirement and you only need to Bulk A. Ras

Basically you bring A. Ras to tank initial hit, Iseria to apply debuff (Target from SOS, Unbuffable, and Def break), then allow A. Lots holding book to CR Push Sigret via S2 which will also apply attack buff. Sigret doesn't need any changes if you have Rage Set and DDJ. Just soulburn S2 and last hit with S3 and Zio will either die or be on 1/4 HP.

With this build, you just have to make sure all the debuffs will land and keep resetting until it happens.