The achievements are unlocking properly now, but they are not retroactive, so if you attempted this before, you will have to do it again.
The easiest way to get this achievement is to wait in the plane all the way to the end of it's path to where it automatically ejects all of the remaining players. There will be several players that are not actually playing that will drop with you.
Just run to them, punch 'em out, then do it again until you get 20.
Before the game went Full Product, everyone at the end of the plane just drifted and went the same way without having to do anything. Now, you have to watch the direction the majority of the group is going in, turn, and drift with them.
If it looks like the plane's flight path is going to end out over water, you may as well leave the game and try again as most AFK players will just float out to sea.
Another way to get this one is to punch out a player you knocked in duos or squads.
If you happen to have four players to punch out (I've had as many as 17 land at the end that were inactive), you will get the Killing Spree Achievement as well.
You will get these along the way:
As VictimOfDesire pointed out, if you are near a house and can pick up some grenades, crossbow, etc. you can get the Pan achievement along with the ones tied to those other specific kill types.
You can also take another player's clothes and grab this one as well:
Update 6/16/2020: With the addition of bots now and a low player count, it's pretty easy to get these in solo squads. Knock a player and punch them out. You can get also get Crossbow, grenade, kills from a car....just about anything you need with this method. Bots are generally dumb and easy to kill.