Am i cooked?

I just defeated HotK in custom mode WITHOUT ACHIEVEMENTS and got my 3rd boss cell. I dont remember BCs droping in custom mode. Of course, i didnt get the achievement. How will i get the achievement now if the HotK wont drop any more 3rd boss cells? Do i really have to make a new save file? For those who are wondering why i have "5BC (completed)" flair i completed it on mobile and just installed Dead Cells on steam.