Firesweep + null C disrupt is the way to go. If you didn’t know, null C basically turns firesweep weapons into dazzling weapons, and you can still counterattack staves, wind sweepers, and other fire sweepers.
C skill is anything youd like, but threaten ATK/SPD ( or ATK/def if you think you can outspeed your foes), and joints are the best there. Smokes and threatens are good budget. For seal, Id use a bulk increase, but it’s also preference
If you really want one of those three however, gaurd sword is best for EP (DMG reduction only does so much against ignis/glacies), Cake cutter is for Player phase (probably best for flashing blade because of the +4 SPD if you really need to proc it on most any unit) and bell ringer is mixed phase (keeping a galeforcer fresh is important, otherwise they’ll just die in their second combat).