Here is the full list of all 21 Super Hydorah achievements.
Score 50000 points or more.
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 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.
Unlock all achievements!