Yes it's a rank sorry to tell you it's a level in first place oTL
You only want to be at rank 30 to get all reward from colabo.
It's easy to reach just run the campaign until you think you can't go any further and start farming around there.
If you lazy you can wait for Sunday. There is daily that focus on leveling your ranking.
But run campaign may more comfortable. Since the daily quest has same system as FKG daily farming quest.
Roentgen ; I never play GS before. I notice the colabo while loitering in FKG in early April.
And I got Anri in early May. Right now the banner and checklist is still alive.
Maybe have to see by other fresh ID because I'm already trade all stuff.