Legion is a fight where consistency is the most important thing. Because of that I will recommend sets without SW for now, but it's possible that SW is actually better. Elze is the only monster where SW would be close to impossible to play so another source of evasion would be necessary. Narga and zino both are more difficult with SW but possible. You'll have to decide for yourself if you want to have more difficult fights in such a long quest.
I'm convinced now that choosing an ice evo weapon outperforms both rondo and the BZ prayer, feel free to disagree and prove me wrong since there have been good runs with them, but in my experience I perform better with an evo. If you want to play B.Z. you can use the set 4.9 from the Set Collection.
The Evo's to choose are Purge Frostsword "Shine" or Nepa Icecutter "Shine".
In this fight I'd heavily recommend playing long length. So play the Length sigil on the evos and rondo but not on the prayer.
Sigil 1: [ SnS Up 15 | Beam Slash | Length/Element 12 ]
Sigil 2: [ Element 15 | Element 15 | Element 15 ]
Sigil 3: [ Element 15 | Element 15 | Element 15 ] or [ Element 15 | Duration 12 | Wait Time 17 ]
You only have to take an Ice Attack Fruit at the start of the quest since shiriagari is built in the set.