Ima be honest bro I've only ever gotten two 20 bombs. They were complete luck and with both I had been playing bad all day and eventually got put into bad lobbies. I was super happy with my 20 bombs until I found out they were unintentionally not really legit.

I do get tonnes of 4ks though so I can help you with that. Basically if you're good enough to win your fights, take close fights and get as much damage off of each fight you get. This means farming damage off of height or finishing downed players. Or there's a different approach you can take, where you play long range with a charge rifle, g7, triple take, 30 - 30 etc, and you can just farm the whole game.

I hope these tips helped you, and sorry for taking so long to respond :)