Here is the full list of all 29 Into the Belly of the Beast achievements worth 1,000 gamerscore.
Defeat The Glutton
Save your first child
There's something in your throat
Collect your share of upgrades
You managed to find all monsters in the bestiary
You engaged in combat with at least 50% of all the beasts inhabitants
Successfully end your playtrough by mostly eathing green DNA (80%)
Why hunt, when tasty food presents itself? Congratulations! Your a canibal.
Defeat at least 100 enemies
Beat the game, without having died a single time. Including checkpoints.
You managed to defeat a boss, but managed to die anyway. Well...
You died the true death two times in succession, on the same stage
You successfully rescued all of your offspring.
Four in One
Two in One
You managed to defeat a boss with only 10% of your own life left.
You managed to defeat a boss without ever taking damage
Find all the hidden treasures
Good Thinking, sir!
Find a way to escape the beast
Defeat the Twins
Three in One
Hard to digest
Manage to end a stage in full health
Beat the game, without restarting a single stage.
Meat an fishballs. End your game by eating mostly purple DNA (80 %)