Here is the full list of all 44 Song of Nunu: A League of Legends Story achievements.
Survive the avalanche with Braum.
Dream for the third time.
Activate Ramshara, the forgotten city.
Find the frozen wild yetis.
Meet Volibear, the Relentless Storm.
Defeat 25 enemies with a finisher.
Dream for the second time.
Awaken the Kellurel.
Win all the snowball fights against Willump.
Hug Willump.
Make Willump dance.
Forge something with magic ice.
Find the Jurlberry fruit.
Three booms in a short time are better than one.
Hit a poro with a snowball.
Meet Ornn, the Fire Below the Mountain.
Enchant a krug.
Enchant an Elnük with the Flute.
Meet Braum, the Heart of Freljord.
Freeze an ice wolf.
Dream for the first time.
Find one stanza of the Notai Song.
Freeze a waterfall.
Win a fight against multiple enemies without ever being hit.
Help one sad Poro.
Throw a snowball into Willump's open mouth!
Find one mural.
Break a snow figure made by Willump.
Free Willump.
Complete the Biggest Snowball Ever sequence.
Defeat Lissandra.
Complete the Epilogue.
Sneak into the Frostguard Citadel without being detected.
Eat a bunch of berries offered by Willump.
Help all the sad poros.
Find all the yeti murals.
Complete the Notai Song.
Activate 5 consecutive climb boosts.
Activate 20 boosts while sledding.
Complete the Ramshara drums sequence without failing.
Defeat 6 enemies using the environment.
Defeat 50 Frostguard soldiers using the biggest snowball ever.
Complete all sledding sequences without taking any damage.
Hit Ornn with a snowball.