Here is the full list of all 21 Super Hydorah achievements.
Destroy Hydorah.
Destroy Hydorah and all his seeds.
Find all hidden secrets in the game.
Unlock all achievements!
Find a floating blue stone.
Find a giant green egg.
Find a jumping flower.
Destroy Hydorah and all his seeds using no continues.
Score 500000 points or more.
Find the chest of Grialia.
Score 450000 points or more.
Score 400000 points or more.
Score 350000 points or more.
Score 300000 points or more.
Find a pink elephant.
Score 200000 points or more.
Score 250000 points or more.
Score 150000 points or more.
Score 100000 points or more.
Score 50000 points or more.
Find an escaping astronaut.