I have every stage cleared EXCEPT for Stage 7 and Stage 6, but I'm fairly sure you don't need Stage 6 to unlock it since I saw a streamer with stage 9 unlocked without any of the desert campaign cleared.
Damn...do you happen to know the compositions for the maps? Google translate can only do so much...
Check the northern front section from the main page if you haven't already figured it out. It has the branching rules. ;;
What do you think?