Can you istantly Active boss cells without needing to go to the First buone?

So ive got the 5ft and last boss cell from the giant, and ive continued my route till hand of the king, i SOMEHOW killed him without talking dmg but ofc cant enter the next room since rn active i have only 4 boss cell and not the 5ft i got before. Is there anyway i can enter the room or i just have to start another route... (Btw this was the build, ive found some other good abilities but decided to keep giant whistle)

So ive got the 5ft and last boss cell from the giant, and ive continued my route till hand of the king, i SOMEHOW killed him without talking dmg but ofc cant enter the next room since rn active i have only 4 boss cell and not the 5ft i got before. Is there anyway i can enter the room or i just have to start another route... (Btw this was the build, ive found some other good abilities but decided to keep giant whistle)