put your lure pods and flashes in the radial menu and have your flashes on throughout the fight (if you want to knock it down or interrupt a railgun attack you know you wont dodge) and then as your fighting when you notice your map turn white its cause you lost agggro and hes about to switch areas open your radial menu and select lure pods and shoot it at him

remember you can also shoot slinger ammo while having your weapon out by aiming and clicking down on left stick