How do sone people have insane cp in challenger world?

I really curious and thought I should ask. Recently with rental gear I can solo normal vhilla and get to about 40k cp with my kanna. I've been doing some parties but sometimes I see a Ren rocking full arcanes partying with like over 100m cp. I got back into the game back in November and I saw this starting at the beginning of the this month January. How do you get so strong so quickly?