Then what would be the best B skill for Ephraim or similar units with low speed but high Atk?
Quick Riposte is the go-to as the first person they tank will eat a 2x if the user's not -breaker'd / Wary Fighter'd, and people like Ephraim have the firepower to end people in those two hits. Note that Quick Riposte doesn't negate an enemy 2x (as in you'll both 2x each other if the enemy's fast enough or using Brash Assault), but Ephraim has pretty good Def so it shouldn't bother him -too- much.
Same-breaker (Lancebreaker for Ephraim) is the second go-to. Low-Spd but high Atk+Def units will always double units using the same weapon if they don't use same-breaker themselves. At worst for a slow unit, a double breaker situation just makes the match even again as they're used to getting doubled and should have the Def to survive. At best, the match tips massively in their favor as a lot of high-Spd units tend to have bad Def or Res.
Vantage is the third go-to. It's Vantage. Can make kills free on defense if the enemy's HP is low enough or you've got a special ready. It's awesome in its own way, though some prefer the ORKO potential of the previously mentioned skills.
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Wings of Mercy and Escape Route have some pretty fun applications both offensively and defensively, allowing the user to warp across the entire map while still being able to attack.
Brash Assault is an interesting choice for slow units, but the HP requirement and the target must be able to counter requirement more often than not gets people to shy away from it.
Rest of the B-slots aren't really worth mentioning, I don't think.