Use only Byleth on Normal/Casual can get you through the battle quite fast.also use NG+ to get exp and support level faster.You just need a bit of grinding early game. When he get snowball on level you just put him in to enemy range and press skip.He will kill every thing that try to attack him. I even beat the game using only Byleth on Maddening/Classic No NG+ (with a lot of DLC grinding,statboot and some clever play.) so I think solo Byleth on Normal/Casual would be fine.Also keep the save file before you go to the last battle so you can pick some one else to S-rank without play all over again.(same to Goddess Tower event you can talk to gatekeeper in that chapter to choose who to appear.) Some characters are playable at specific routes so you might have to play all over again more than one time.
(and sorry if my gramma was bad.I am not a English native.)