Grand Hero Battle - Clarisse

Raventome Sophia with swordbreaker walls out the entire cast with the exemption of the blue mage. Draw all units into the bottom left corner making sure to keep the bmage out of range. Use a dancer to allow Sophia to score blows and move out of the way of the bmage. +def Sophia is optimal if you don't want to use a healer. You can snipe the bmage with a distant counter (not lightning breath unfortunately due to watersweep).

1st+2nd turn: You move Wrys and Jaffar at bottom left along with Ninian, Wrys should be in the tile that can break the wall and Jaffar left of him, Ninian can be in bottom right one, Xander above her. 3rd turn: Wrys breaks the wall, Xander goes to the defensive tile and hits red cavalier, Ninian moves 1 space up (out of harm's way ofc). 4th turn: Xander goes back to Wrys, Wrys heals him to full, Ninian dances Xander and he goes to his former position (the defensive tile). 5th turn: Xander goes to Wrys, Wrys heals him to full and end turn. 6th turn: Wrys heals Xander, Jaffar kills red cavalier (Xander might have already killed him), Xander kills dagger user (he has charged bonfire),Ninian dances Xander and Xander repositions Ninian to safety from last sword user. 7th turn: kill sword user with whatever you want.

Xander: Weapon: Siegfried, Assist: Reposition, Special: Bonfire A: Fury 3, B: Quick Riposte 2, C: whatever (you could use savage blow 3 to make it easier), Sacred Seal: HP+3

Any healer with Rehabilitate

Any dancer

You don't even need a 4th unit actually, just get a ranged blue/red/archer/rogue to damage or kill the 2 sword users.

Xander, Camus 5*. Ursula, Frederick 4*.
Xander, Ursula w/ Hone Cavalry. Camus w/ Goad Cavalry. Frederick w/ Fortify Cavalry.
Ursula w/ Draw Back, Blarblade (3* version is fine). Camus w/ Vantage, Swap, Moonbow. Frederick w/ Reposition, Wings of Mercy (2 is plenty). Xander Bonfire, Swap, NO Fury; empty A slot is fine.
Xander HP Seal, Frederick Spur Def Seal.

T1 - Xander receives Hone and Fortify buffs. Place Xander to the bottom left of the breakable wall with Frederick below him, Ursula to the right of Frederick and Camus two spots right of Xander. The green mage should kill itself on Xander while Clarisse breaks the wall.
T2 - Xander uses Bonfire on Clarisse, Ursula finishes Clarisse off. Frederick Repositions Ursula south, Camus Swaps with Xander. Xander should barely survive the blue mage attack thanks to his HP Seal.
T3 - Camus kills the blue mage. Ursula damages the sword soldier. Frederick Repositions Ursula. Xander retreats for sweet life.
T4 - The sword units should have suicided on Camus by now. Have Camus damage the thief, then have Xander Swap into Ursula so she can reach Camus and use Draw Back. Frederick uses Wings of Mercy to jump in and finish the fight.

Units:
Zephiel 5* with threaten attack 3 and Bonfire
Bridal Cordelia with brave bow+, Lnd 3 and Luna
ANY dancer/singer
4th unit not needed
Turn 1:
Use Zephiel to break the obstacle from the right and let him tank.
Turn 2
Use B cordelia to kill the sword cavelier and use zephiel to kill the dagger user. dancer/singer to give zephiel an extra turn to move onto the defensive terrain and kill the green cavelier.
Turn 3:
Use B cordelia to kill clarisse and dance/sing zephiel to let him attack the blue mage twice and win the GHB

Xander with Quick Riptose, reposition, Clarine with Fotify Horse, goad horse for two another. Let Xander and Reinhardt kill all. Ez GHB.