Premonition: Invisible Ties
Condition: Defeat Boss
(Does not move)
As it is a prologue level, there shouldn't be any difficulty, even on Lunatic mode. Get used to how moving units around works, as the boss does not move at all, and so you can pace yourself. Feel free to get used to the menus, the way Player/Enemy phases work, etc.
If it is your first time, small tutorials will pop up on the bottom screen, the images alone should give you the gist of what is being said. When you're ready to go on, it may be helpful to just pair up Avatar with Chrom (select Avatar, then move next to Chrom and choose "Pair Up").
Note that Pair Up is disabled on Normal Mode, until a certain chapter, but you can enable it instantly by going to the options menu and switching off Tutorial.
After that, just move Chrom and your Avatar (or just one of them if you paied up) towards Validar on your first turn. Then on your second turn, move them adjacent to Validar and select "Attack".
To finish off Validar easily, make sure you've paired up and Chrom is the active unit (if not, select "Switch" immediately after moving) and then press X or Y to switch his weapon from his initial Rapier to the Silver Sword, before fighting.
Once Validar falls, the chapter ends.