Ballad of The Duke of Velthomer

-Azura-P
atk +spd
Distant Counter (DC is not mandatory, but do not use Triangle Adept)/WoM/Drive Res/Luna

-Arvis
Warding Blow (use anything else)/RecoveRing/Def Ploy/Draw Back/Iceberg

-Lute
+spd -hp
Fury/Vantage/Res Ploy/Atk Ploy seal/Draw Back/Iceberg

Sonya
+res -spd
Mirror Strike/Vantage/Res Ploy/Draw Back/Iceberg

Sacred Seals:

Only Atk Ploy 3 on Lute is really mandatory. Use seals like Q-Pulse, Atk Smoke and Savage Blow on the others if it suits you.

There's no perfect positioning strategy since any movement can cause the enemy units to go to wildly different spaces, but you should always start the battle off by attacking the manakete with Arvis, dancing him with Azura, and then having the former position Azura to safety with Drag Back. From there, move Arvis to the right to manipulate the AI, and the manakete should show up on the right chokehold (put Lute at the bottom so as to trigger the ploys on said manakete). From there, kill the manakete with Arvis and pick off units as they come. Some small tidbits from here on out depending on what happens:

-Sonya can tank the cavalier twice if needed.
-In a desperate scenario, Azura can weaken Oliver for Lute or Sonya to take out.
-The green mage has no hope of killing Arvis at all, and thus the latter can be used as bait for a long while if needed.
-Azura-P can tank the red mage once if needed. This is why you don't run Triangle Adept on her.

Arvis

Lute

Sonya

Performing Azura