It's supereffective: Deal a critical hit using a bow or wind magic on a flying unit.
Hey!, I'm RARE: Attack Yarne before recruiting him.
NO DEAL: Discard a Bullion
If it ain't broke...: Forge a weapon with one use left.
Ka-ching: Have a thief/ assassin/ trickster open a chest with Bullion (L).
Caution, children at play!: defeat a level using only children characters.
Written in the stars: Defeat a boss using Astra.
Unstoppable: Activate Lethality/ Astra/ Ignis on both Galeforce turns.
Banana Peel: Read a support in which Sumia trips.
Sky Wars: Attack an enemy flying unit over water/ sky with a flying unit.
Air Forced: Use only Bows and Wind Magic in paralogue 17.
But I wasn't eavesdropping!: In the Barracks, see the same character on the top and bottom screens.
Bam! Instant forest fire: Attack using fire magic from the woods.
Tastes like chicken: Defeat a Griffon Knight using fire magic.
Sticks and stones: Give a twig/ log to a manakete/ taguel.
I'll kill you twice!: Defeat a risen with Sully.
MY PRECIOUS: Let Gaius use the Gaius Confect.
Torn Binding: Break a tome.
Not for Karate: Break a staff.
Bound to happen anyway: Break any weapon that is "weak and ill suited for battle".
Pointy: Defeat an enemy in one hit using a glass weapon.
They're magically...: Have a character hold, but not use, a talisman for the duration of a single battle.
Rare Candy: Level up a character paired with Gaius
Your'e my hero: Dual guard a base class unit with a Hero.
Made in Valm: Find the boots. (I didn't )
Spotlight stolen: Achieve maximum support between Chrom and the Avatar.
Oh I just can't wait to be king!: Promote Chrom.
Shnazzy: Let Fredrick fight with silver weapons only, for the duration of the main game.
Here we go!: Complete a skirmish using Sully, Stahl and princess Maribelle.
The incredible Gregor: Promote Gregor
Dragons in dungeons: Complete an indoor battle or skirmish using at least two manaketes.
Don't need no healers: Fully heal a unit from less than (3/4) health with Lifetaker.
They never learn...: Have a unit sustain lava damage three times without healing.
There's a curse for that: Let Henry and Thraja use dark magic on the same enemy.
4 chords: Let Olivia (as a dancer) defeat an enemy with exactly 4 hits from a brave sword.
GASP, it dosn't work?!?!: Activate Lethality on an enemy with Dragonskin.
Clingy: Keep Sumia paired with Chrom for the entire chapter in which she first appears.
These are not the Lords you're looking for: Defeat less than five of the 12 deadlords.
Sorting Hat: Reclass Laurent into the same class as Miriel is currently in.
I have more in mind but you people will probably get tired of reading them, so I'll stop here.
I tried to make these accessible for all difficulties.