Complete all challenges for 5 weeks
Complete all the challenges for five given weeks to unlock the fifth of the seven challenges. Once you've got your loading screen, head to quadrant H10 and climb up to the top of the wooden grab, right on it's head. There you'll find your battle token!
If you look at the loading screen you unlocked, you'll be able to see the battle token above a tiny crab on the right side of the image.