Solved. Please lock. Thank you :)

You probably need this to unlock the 7-headed one. [item2=Silver Panzer Hydra Medal]

"Take a break from dancing and battle The Panzer Hydra!

Remember, to battle the Panzer Hydra you’ll start off against the single-headed version, and if you defeat it ten times you’ll earn yourself an achievement and a medal that you’ll use to unlock the three-headed version. Defeat that one ten times to get the medal to unlock the next, and keep doing that all the way up to the final eight-headed Panzer Hydra!"

Were you able to obtain that medal which unlocks the next one? It's given by the achievement, and is a tradable item which some user might have sold without realizing that they might need it later.

I didn't get the achievement or a medal.

I see the 7-headed one in your battle records now.

Glad you figured it out!!