So Athos can't use any magic. Fire Emblem magic is stopped by the "silence" staff/status effect. DDS and FF magic is stopped by "mute" so therefore no speaky no magicy. At best he can use Agahnim's stuff.
So Seph wipes the weak mages on the other side asap. Seph CAN hit Cielo cause he has teleportation and limited flight himself. He'll keep up with him-- then Golbez blindsides Cielo with magic and Seph wins.
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No problem!
This is a cute and pop genocide of love!