+Atk / -Spd Ephriam... Suggestions?

Deathblow +eff Siegmund, Aether heavy blade, qr for B skill. +atk Ephraim is best Ephraim. it will work beautifully.

Normal Ephraim?
Mine is Repos, Moonbow, Fury, QR, & Hone Spd with special refine. His damage output is lower than Deathblow variant, but he can bait & kill non-magic red on player phase, & since I always have a healer, it's quite effective.

That... is the best IV for Ephraim, congratulations my friend! :)

Spd is largely an irrelevant stat for him because it is so low in the first place and Siegmund's passive lets him double on initiation regardless of speed difference. As the others have said, slap on a Death Blow with Heavy Blade seal, Aether and Vantage/Renewal and he'll be pretty sweet to go