To unlock the Tip o' the Hat trophy in Far Cry New Dawn, you will need to find an enemy that is wearing a helmet (level 2+), and use bare hands or any melee weapon such as a baseball bat to knock off their helmet.
Approach an enemy that is wearing a helmet, make sure you have bare hands or a melee weapon equipped by holding L1, hold R2 to charge the attack, aim towards the head and then release R2 to hit them and knock off the helmet.
Tapping R2 will not work. You must hold R2 for it to be a charged attack.