|+Atk/-Spd = 53/54/22/32/20
Refined '"`UNIQ--item-884--QINU`"' only activates a double attack if Ephraim's HP ≥ 90%. Therefore, to keep his HP high, '"`UNIQ--item-887--QINU`"' and '"`UNIQ--item-890--QINU`"' are taken to mitigate the damage taken from the counterattack between his two attacks. Pavise will start on a 1-CD from Shield Pulse, and will be charged after Ephraim's first attack. It will then block 50% + 5 damage, making him take very little to no damage at all from many enemies.
Math time. Even without an offensive special, he does great damage due to '"`UNIQ--item-893--QINU`"' giving him 60 Atk on initiation (58 with '"`UNIQ--item-896--QINU`"'). If another blue lance unit has 35 Def, they will still take 25 damage twice, meaning they need over 50 HP to survive. This goes up to 72 effective Atk against reds, which is enough to kill Arden not considering buffs or passvies. Also, Ephraim can lose 5 HP and stay within 90% HP at 53 HP. If the lance unit aforementioned has 52 Atk, then they do (20*0.5)-5 = 5 damage, which won't be enough to drop him below 90% HP. With Sturdy Blow 2, increase that to 56 Atk. That's amazing.
Ephraim's C-slot is highly flexible due to his weapon and stat spread. '"`UNIQ--item-899--QINU`"' and '"`UNIQ--item-902--QINU`"' seal (HP +3 works as well) can be taken to improve his solo dominance by ploying enemies and helping the threshold for Siegmund's ability. However, a second purpose of this build could be supporting allies. '"`UNIQ--item-905--QINU`"' and '"`UNIQ--item-908--QINU`"' seal (or vice versa) help defensive units. This route grants them +4 Atk/Def/Res, which will help them tank and possibly kill with Quick Riposte. If offensive player-phase allies need buffing, then '"`UNIQ--item-911--QINU`"' and a different seal can be chosen.
For his Assist skill, the usual '"`UNIQ--item-914--QINU`"' can be taken, or if Ephraim's buffs are being panicked, then '"`UNIQ--item-917--QINU`"' is a another option.
Note: If Ephraim is Summoner Supported, then the '"`UNIQ--item-920--QINU`"' seal should NOT be used. This is because from 50 to 59 HP he can take 5 damage and remain ≥ 90% HP. To take 6 damage and remain ≥ 90% HP, he needs 60 HP, which can only be achieved in this build with a Summoner Support, HP +5 seal, and a +3 merge. Instead, an '"`UNIQ--item-923--QINU`"' seal will work wonders, raising his Atk to 59 (65 on initiation).