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