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