Prologue: The Verge of History
Condition: Defeat Boss
(Does not move)
General Strategies for Normal/Hard
Bunching yourselves together defensively and moving as a group is your best bet for this map, especially on the harder modes. Frederick is nice to use for this purpose.
Pair up your Avatar with Chrom and making sure Lissa is always out of enemy range, and if she can't be, then paired with Frederick for protection. Use the obstacles on the map as a way to make up for spaces you can't occupy with your units!
Heal when needed and safe, and proceed very slowly to the bridge, and cross when you're sure you'll be okay!
Like Validar before, the boss doesn't move, so use that to your advantage.
General Strategies for Lunatic
Frederick is your friend! I recommend pairing your units up, Frederick will be able to protect Lissa this way, and Chrom and your Avatar should preferably avoid trouble. Mages are a great danger to most of your units, so prioritize wiping them out when possible.
At this point a lot of units are a danger to anyone but Frederick, so you may have to use him as your main unit throughout; if he gets in danger, be sure to use vulenaries.