First, go to stage 6-10 and see Lir. She will give you quest to meet Kali in stage 7-1. After you accept the quest, go to see Kali in stage 7-1 and complete the guest. Kali will give you guests until you will not complete all. After you complete the guests you can go to stage 7-2.
Kali appears as a NPC here.
All the monsters in this stage will be called with the names "Destruction".
Also, this stage is good for making money.