Here is the full list of all 41 Majin and the Forsaken Kingdom achievements worth 1,000 gamerscore.
Go to the true ending
Defeat the "Creator of All Darkness"
Open all Treasure Chests
Get all costumes
The Majin's "purification" ability rises to maximum
Raise Friendship level and Tepeu's level to max
Defeat 300 enemies with combination attacks
Defeat over 10 enemies with a flame finish attack
Defeat over 10 enemies with a purification finish attack
Defeat 5 or more enemies at once using a finish or combination attack
Defeat over 10 enemies with a wind finish attack
Defeat more than 50 enemies with finish attacks
Defeat over 500 enemies
Have the Majin revive you 3 times.
Defeat Ixtab
The Majin's "strength" ability rises to maximum
The Majin acquires "purification" ability
Defeat 50 enemies with sneak attacks
The Majin's "lightning" ability rises to maximum
The Majin's "flame" ability rises to maximum
The Majin's "health" ability rises to maximum
Defeat over 10 enemies with a lightning finish attack
Defeat Tlaloc
Get 3 full costume sets
The Majin acquires "flame" ability
The Majin's "wind" ability rises to maximum
Help the Majin out when he's in trouble 30 times.
Defeat B'alam
The Majin acquires "lightning" ability
Become "haunted by darkness" 10 times.
Feed the Majin over 10 Blue Fruit
Help the Majin out when he's in trouble 10 times.
Defeat Caprakan
The Majin acquires "wind" ability
Defeat over 100 enemies
Defeat over 30 enemies with traps
Attack shielded enemies 100 times.
Give the Majin more than 500 orders
Escape from the Dark Castle
Talk to the Majin 100 times.
Save the Majin