|+Atk/-Res = 45/59/25/34/17
Ephraim: Restoration Lord can use his legendary version's amazing weapon to accentuate his high Atk & low Spd and become a serious dual phase threat. However, this weapon makes him a bit of a lone wolf, unable to retain his potency while receiving combat buffs. This build makes Ephraim more self-sustaining and brings out his potential. Let's break it down...
Weapon: Flame Siegmund is taken because it's effectively a 19 Mt weapon and allows Ephraim to double foes on both phases despite his low Spd. The only constraint here is that Ephraim cannot double enemies when he has more allies than foes within 2 spaces. To help make sure he isn't too close to his allies, he needs the right Assist skill...
Assist: Smite is the perfect assist skill for Ephraim. If he needs to get in front of a squishy to tank on the Enemy Phase, he can run in front of them and Smite them backwards to put them outside of the Flame Siegmund effective area.
Special: Aether is a good special for this build, as it increases Arena score and can charge quickly due to Steady Breath in the A-slot. Ephraim will appreciate the healing as well, since he will probably take a good bit of damage due to being doubled by nearly all foes.
A Passive: Steady Breath is the premier enemy phase skill, since it boosts Def and charges the special so quickly. This is what sets Ephraim apart from Ephraim: Legendary Lord, as his cavalry counterpart cannot learn Steady Breath.
B Passive: Def/Res Link 3 is great for Ephraim, since he will likely be using Smite to isolate himself. Also, if a unit Repositions him to let him charge into battle, he will also get the +6 Def/Res. If these buffs are achieved via Tactic skills or otherwise, then Def/Res Link should be swapped for Guard 3 to help protect Ephraim even more.
C Passive: Since Flame Siegmund does not have the Atk buff of Siegmund, Ephraim doesn't have to take a support C Passive. This slot is Flexible and can be used for various functions. A popular pick for this type of build is Atk Smoke 3, which is very effective since Ephraim will be around multiple foes for Flame Siegmund to shine.
Sacred Seal: Attack/Def +2 works well for mixed phase units like Ephraim, but if you need a higher Arena score option, Close Def 3 or Atk Smoke 3 will do nicely as well. Ultimately, this slot is Flexible and should be filled according to team needs.
This build will kill almost any ranged unit found in Tier 19 and 20 in the player phase, and will beat most reds and blues in the enemy phase. With the bulk and damage of a Summoner Support added on, Ephraim will work wonders.