Here is the full list of all 20 Super Hydorah achievements worth 1,000 gamerscore.
Find an escaping astronaut
Find a giant green egg
Find a pink elephant
Find a jumping flower
Find a floating blue stone
Find the chest of Grialia
Destroy Hydorah
Destroy Hydorah and all his seeds
Destroy Hydorah and all his seeds using no continues
Find all hidden secrets in the game
Score 50000 points or more
Score 100000 points or more
Score 150000 points or more
Score 200000 points or more
Score 250000 points or more
Score 300000 points or more
Score 350000 points or more
Score 400000 points or more
Score 450000 points or more
Score 500000 points or more