To jump to approach 2: (2:59)
Sadly I can't make this work with only 4* units without some skill inheritance.
Approach 1 needs 4 SI
Approach 2 needs 2 SI but requires you to have a second Palla.
Approach 1 (4 SI)
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Units used, skills inherited and seals used (all IVs neutral):
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Narcian: Inherited: Atk+3 | Seal: Spd Smoke 1
Cordelia: Inherited: Smite | Seal: Atk+3
Palla: Inherited: Spd+1, Lunge | Seal: Spur Atk 1
Subaki: Inherited: N/A | Seal: Drive Atk 2
Otherwise, they are using their default skills
Note that a -Spd Palla, will not work with this approach. A -Res Cordelia might work if you use C rank Ally Support with Narcian
Approach 2 (2 SI)
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Units used, skills inherited and seals used:
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Narcian (neutral IV): Inherited: N/A | Seal: Atk+3
Cordelia (neutral IV): Inherited: Smite | Seal: Savage Blow 3 (but Savage Blow 1 should suffice)
Palla 1 (neutral IV): Inherited: Lunge | Seal: N/A
Palla 2 (+Def -Spd): Inherited: N/A | Seal: Drive Atk 2
Otherwise, they are using their default skills
Palla 2's IV shouldn't matter too much. If the enemy moves in the same way, you can see about using Spur/Drive seals on Palla 1 to bolster her bane.
Summary
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Both variations operate on a simiilar premise, but the execution varies a little for the enemies other than Xander and the Blue Lancer.
Both approaches relied on Cordelia launching Narcian forward with Smite, and him drawing the enemy forward by slipping next to the Blue Lancer.
A combination of Drive, Goad and sacred seals are used to allow Cordelia to take out Xander in turn 2.
The positional effect of Lunge is used to get around the issue of the needing multiple attacks to take an enemy out but lacking the tiles to do so.
Other notes are in the subtitles