How’s a -def +Attack Winter Ephraim?

he's still good, the atk superboon is especially good

Of course. If used correctly he should reach 67 Atk and 39 Def with Atk/Def Solo, and with the activation of Bold Fighter he should be getting attacked only once every initiation anyway.
Not satisfied? Then take armor buffs in consideration. He can still reach 45 Def with Fortify Armor, and can stack even more with Ward Armor.