Main Quest: Garreg Mach Monastery

Before starting the main battle, choose Strategies. Each has a point value and can provide advantages. 'Persuade Hapi' and 'Persuade Constance' cost 10 Strategy Resources each and allow you to recruit new playable characters. 'Impregnable Wall' can also be helpful due to in-battle events. 'Impregnable Wall' and 'Blaze' unlock via Surveying Spots in other regions.

Position your troops: Place Bernadetta in the northern batch for upcoming flying units, and Shez in the bottom half. Edelgard, in her Armored Lord class, should be in the northern section to break cracked walls and reach a stronghold.

Recruit Hapi and Constance using the 'Persuade Hapi' and 'Persuade Constance' Strategies. Hapi is in the stronghold south of Shez's starting position, and Constance is in the first stronghold on the northern end. When the Strategy notification appears, hold the button to initiate the Strategy, which changes the mission to defeating that enemy unit for recruitment. Switch characters as needed since they are on opposite sides of the map.

Capture all nine strongholds.

Keep a magic user or two back at the central stronghold on the western side of the map for when Armored Knights appear.

Continue the mission, alternating between northern and southern paths.

When Flayn's event appears, cut through cracked walls with an armored unit and defeat her to stop the Resonant Lightning attack.

Prepare for reinforcements: Pegasus Knights in the north (send Bernadetta) and Cyril in the south (Shez can handle him).

Avoid taking too much damage by managing ballistae/fire orbs.