I’m a B!Claude enthusiast. After using him a bit, he’s on my +10 unit list along with B!Edelgard for the (likely) Hero Fest in 5ish months. I also use him for Astra, and I haven’t completely decided which Mythics to run with him yet, but they should both work. And he has B!Lucina strapped to him two spaces behind at all times. She runs double speed drives and Even Pulse Tie.
Anyway, I currently use Luna (Moonbow outside of AR), Close Counter (I prefer retaliating against dragons to the stats from Foil, but Foil is also great), Quick Riposte, A/S Rein, and Iote’s.
The Riposte is Impact insurance, but also helps against extra-speedy units since there’s no speed-boosting Astra Mythic yet. I’m not convinced it’s his best B slot, but the options are all “meh”. I’ve used Vantage too, and it is also nice insurance if he happens not to heal up all the way, but honestly it didn’t happen very often.
When I get Mystic Boost fodder I want to try running him with Fury 4 and Mystic Boost to really pump his stats up, but I’m not sure it will be worth not retaliating to melee units.
As far as Pulse Smoke goes, it would be great on him. I’m not sure it’s needed if you have a Pulse Tie skill on your B!Lucina or if you have a team dedicated to Infantry Pulse defenses. The A/S Rein is amazing too, so I’m going to try to keep it on him if I can, but IP teams are one of his only weaknesses, so the Smoke might end up being better, I’m not sure.