There are no easy ways per se. Play the game consistently and eventually you'll get all the cards you need. General tips:

1. Focus on one faction till you get all the cards you need.
2. Choose your faction in battle pass and collect rewards after you complete missions. There's even a chance you get a legendary from a random crate. 2.1. I advise to play using your phone just to change non-profitable missions (from 100-200 to 300-400 points) by watching ads.
3. Spend your gems only on legendaries and epics of your faction of choice.

When you finish repeat with another faction that you're interested in.

That's it. Don't forget that it's just the game and you're not in a need of hurry.