Hi Lawrence,
Kingdoms Arms Race is available through the event centre. It is set from 1 to 5 stars. The more stars, the more points and rewards. There is a few videos on how to do it on both the Gunmuse and Esqua YouTube channels.
Essentially the trick to getting 4 and 5* challenges and rewards is to do the level below and in a decent time. It also sometimes means hanging around the 5* for the countdown to finish, instead of leaving the event centre and waiting. I’ve had a few times where I leave the KAR page and the 5* challenge that had been counting down to be revealed, disappears.
I hope this helps. Good luck.
Keelah k410