Here is the full list of all 84 LEGO Harry Potter Collection achievements worth 2,000 gamerscore.
Complete the game to 100% (Single Player Only)
Defeat every Harry freeplay variant as Lord Voldemort
Use a Weasley box with every Weasley
Unlock ALL of the Order of the Phoenix character variants (Single Player Only)
Unlock all witch characters (Single Player Only)
Unlock every pyjama character variant (Single Player Only)
Unlock every bad wizard (Single Player Only)
Unlock all of the members of Dumbledore's Army (Single Player Only)
Unlock every character with a pet (Single Player Only)
Defeat 30 Red Caps
Complete the Bonus Level
Defeat Bellatrix with Neville (Waiter) in a duel
Defeat Harry (as Voldemort), in the Graveyard
Defeat Quirrell in the Quirrell boss fight, using Voldemort
Complete the game to 100%
Buy every version of Voldemort (Quirrell/Tom Riddle/Voldemort)
Collect all of the Gold Bricks
Rescue all students in peril
Unlock all Animagi
Hide in a barrel as Snape
Buy the entire Gryffindor Quidditch team from Book One
Buy the entire Weasley family
Collect all of the house crests in Year 4
Collect all of the house crests in Year 3
Use the Time-Turner
Collect all of the house crests in Year 2
Collect all of the house crests in Year 1
Complete the Quidditch level within five minutes
Collect 1 billion studs (Single Player Only)
Defeat Bellatrix as any Sirius Black character variant in a duel
Complete the scene where Hermione and Harry dance in the tent
Unlocked on hitting 50% game completion (Single Player Only)
Complete Year 8
Complete "The Flaw in the Plan"
Complete "Snape's Tears"
Complete "Fiendfyre Frenzy"
Complete "Burning Bridges"
Visit the Hogwarts Foyer in Year 7/8
Complete "Back to School"
Complete "The Thief's Downfall"
Complete Year 7
Complete "DOBBY!"
Complete "Lovegood's Lunacy"
Complete "Sword and Locket"
Complete "In Grave Danger"
Complete "Magic is Might"
Complete all lessons (Single Player Only)
Complete "The Seven Harrys"
Use the Deluminator
Complete Year 6
Complete "Horcrux and the Hand"
Complete "Felix Felicis"
Complete "Love Hurts"
Complete "A Not So Merry Christmas"
Complete "Just Desserts"
Complete "Out of Retirement"
Complete "A Veiled Threat"
Complete Year 5
Complete "A Giant Virtuoso"
Complete "Kreacher Discomforts"
Complete "Focus!"
Complete "Dumbledore's Army"
Use a Quibbler dispenser 25 times
Complete "Dark Times"
Defeat 10 enemies with a Muggle character
Complete all the LEGO Builder tutorial levels.
Destroy 50 Devil's Snare plants
Scare 20 students using a ghost character
Collect all of the Red Bricks
Defeat the mountain troll, using Quirrell
Turn the sound down to 0 and music off in the options menu whilst in the library
Get True Wizard in every level
Complete Year Four Story Levels
Complete the Story Levels in all four years
Collect 10 million studs in one level
Complete All Story Lessons
Complete Year Three Story Levels
Freeze 20 characters using Glacius
Complete Year Two Story Levels
Defeat 20 spiders with Ron
Defeat 20 enemies with Fang
Complete Year One Story Levels
Collect 500 ghost studs
Knock over ten characters using a ride-able object