Which refine for a +hp -spd regular Ephraim?

Flame Siegmund is not really worth it on him, because it's pretty difficult to meet its follow-up requirements on an infantry unit and if you would be able to meet them, he would most likely just die.

His regular weapon alongside its refine is much better. You are keeping that hone attack in his weapon, which makes him special and you also get a guaranteed follow-up at 90%+ hp, which you can either abuse with healing mechanics or just take that one good fight and call it a day.