Note: Tips, enemy units information, and best team suggestion below are based on expectation currently since the event is not available yet.
Map
A tank is necessary!
Since it is almost impossible for you to take the first due to the terrain, a character who plays a role of tank and survives the first attack is needed. Basically, a character with Def: 30 and Res: 25 or above is recommended.
An attacker should have long range or good mobility
Only a character with long range or good mobility can play a role of attack in this mission. A cavalry or flying unit can easily find a chance to attack a foe and avoid getting attacked.
Use heroes with "Bow"!
Narcian is a flying unit that is weak against an attack by bow, so a character who use bow as their weapon seems to be necessary to a team.
Foes' stats and skills
Narcian (Wind / Axe / Flying)
HP
Atk
Def
Spd
Res
48
37
28
24
22
Weapon
Emerald Axe+ Gives Atk +20% if weapon-triangle advantage, Atk -20% if disadvantage.
Special
Vengeance Grants bonus to damage dealt equal to 50% of damage suffered.
Passive B
Lancebreaker 3 If unit's HP > 50% in combat against a lance user, unit makes a follow-up attack and foe cannot.
Passive C
Savage Blow 3 If unit initiates attack, foes within 2 spaces of target take 7 damage after combat.
Bow Fighter (Gray/Bow/Infantry)
HP
Atk
Def
Spd
Res
41
32
24
20
13
Weapon
Brave Bow Spd-5. Attacks twice if unit initiates combat.
Special
Reprisal Grants bonus to damage dealt equal to 30% of damage suffered.
Passive B
Death Blow 2 Grants Atk+6 if unit initiates combat.
Passive C
Savage Blow 3 If unit initiates attack, foes within 2 spaces of target take 7 damage after combat.