My tip is do mission 8 and have 3 med bots an when ur on bottom of map with all three bots running you are invincible preety much. Then after 5 mins u get like 3zc I think can't remember haven't played for like a month. But repeat this process for like 1 hour to get about 36zc. So play 3-4 hours a day then u get about 100zc daily do this for 30 days then u have 3000 zc then u can get bunch of stuff I used to wait each VIP(monthly) realease then spend my zc.