Try to familiarise yourself with the map and with how tall guardians appear to be at varying distances. My sniping has been improving a lot after I started taking note of where enemies will tend to pop up from and where their head is gonna be. For most kills I don't really need to adjust my aim for the enemy's head, they'll just pop up where I'm aiming at. Also, if you're facing another sniper and you lost the first exchange, don't go back to the exact same spot unless you're significantly more skilled than he is or you know he's gone. The enemy will react much faster when he's already hardscoping where you're gonna pop up from.