Splitgate

The best mode to play for this is ShottySnipes as you will always spawn with both a shotgun and a sniper rifle. You can press by default to swap your weapons. The sniper is the one where if you ADS () you have a scope.

Headshot kills will as far as I have seen always kill the enemy in one shot when using the sniper rifle. I recommend trying to set yourself up somewhere high with a large line of sight so you won't get shotgunned by the people on the ground but will also be able to see a large area.

UPDATE: Since the beta ended and Season 0 began, there is now currently a permanent gamemode called Big Head Snipes.

You can find this under Casual > Rumble.

This is easily the best way to get Sniper headshots. If you don't fancy that then the next best way is of course Shotty Snipers, but I wouldn't worry about this achievement too much as I'm sure you will get this naturally just playing, the sniper is in a lot of other game modes also.

The simplest way to get this achievement is just to add Shoty Snipes to your queue, this will mean you will always spawn with a sniper.