I've been told to concentrate on COZ what is that?

CoZ is the Clash of Zones event that you can find in the Events menu. Each week you will compete against another state for points for a different target each day. (4 weeks in each season against individual states and 1 week in between seasons where you 'warm up' but still do the challenges most days. Completing the challenges is a good way to earn rewards in the game.

Check the guide page for Clash of Zones for more details on each days events and check your Events menu to see what Clash of Zones activity is happening for you.

Focus on the coz tech. Max it so you can get 10 boxes per day. That is by far the fastest way to grow in the game.

MadKing state 65