This is getting even more interesting as I play. When I started Lunatic, I thought the game was insanely difficult, Then I started Lunatic+ and said the same, After managing to beat chapter 4 on lunatic+, I realized my ability to deal with rng and tactics got significantly stronger.
For example, In Chapter 3, after you deal with the first wave and you move to the upper wave, and after killing two knights; you can send a cavalier to the opposite side of the map which you want to attack from. Put him/her right next to their aggro zone. Once you aggro them from your attack side, three of the enemies will move towards the alone cavalier.
Next turn, you will have two or maybe even two enemies to deal with at the attack side(Assuming you've killed the first enemy you've aggrod). While doing this, you take cavalier south and move it towards your attack side. After you killed the 3 enemies(mercenary, fighter and archer) there should be approximately three turns until the trio at south arrives.
By the time they arrive, you should be able to put robin/frederick into a forest tile.
This strategy for Chapter 3 worked on Lunatic+ so much more than I've imagined.