This depends on practice and time, to select the grenade you must press
to throw, as Call of Duty you can also cook
the grenade that is about to advance the explosion, for that you must be in the team from Axis
and look for players on the USA
team who were spawning in the parachute sky, take the grenade and wait for it to descend a little bit then hold
to cook the grenade and you will notice that the grenade makes a sound
indicating when it will explode, when you are sure, throw and as soon as you kill the enemy in the air the achievement will be unlocked.