Cai Chapter 7: Retreat and wear enemies down, then surround the archer bossMinor spoilers
Remove a heavy shield that lowers Attack Speed. Use Short Spears, healing, and retreating to reduce enemy numbers, then trap the boss with three units and an obstacle.
Original video: Fire Emblem: Fortune's Weave—Cai Chapter 7 story-map strategy | Bilibili creator Xianyun SS
For 
Cai's Opposed Agendas, retreat, heal, and chip away at range to thin the enemy army before surrounding the archer boss.
Preserve Attack Speed when equipping core units
Deploying more people is not automatically better. Leave out anyone who would only watch, and organize your main units' equipment. 
Da Mina Spear is recommended as a regular spear. In particular, do not casually equip Cai with 
Wooden Shield: its Weight lowers the Attack Speed you want to exploit here. Bring 
Short Spear alongside a melee spear so you can weaken enemies before they reach you.
Give 
Priest a healing staff. You can also visit 
Blacksmith from preparation to upgrade equipment. Prioritize the main units' usual lances and Short Spear when materials permit. Leave weapons short on materials alone rather than going out of your way for one upgrade.
Stay near healing while attacking at range
As long as you do not rush forward, the enemy has a harder time dealing with you. Mark the archers' range, keep other units outside danger, and use Cai's ranged options. Injured units retreat, and the army stays close enough for timely healing. If an enemy withdraws after attacking, let it; chip it again with Short Spear next time.
When facing 
Hong Hua, first draw out that gambit use, then pull back. Without long-range healing, retreat to somewhere you can survive the next hit and use recovery items. If a tested position gets you killed, move one space and rearrange.
Retreat and counter to reduce enemy numbers
If you cannot win head-on early, wear enemies down through defensive counters. Use terrain to draw them in. When they split toward both sides, withdraw first, reposition to chip HP, then find places where counters can kill. Once only two enemies and the boss remain, turn back for a direct fight.
Surround three sides to restrict the boss's gambit
When the archer boss approaches, place three units on three sides and use an obstacle to block the fourth. It can no longer move.
It retains ways to hit nearby units, but struggles to use the gambit that requires two spaces of distance.
Keep checking attack previews after surrounding it. Attack in turns when you can survive.
This map has no turn limit, so there is no need to rush. Wait when necessary, restore HP and positions, then finish gradually.