So warning this is not that good of a guide this is just what I have done twice so I don't know too much.

First: you want to have a grease gun and a rifle/sniper. The grease gun will be used on the first phrase of the boss, if you are good at aiming you can use other guns, but if not fast fire might work better. You want to attack when the shield is moved and you have a shot at the body of the Riot Boss. You want to fire as much shots as you can to deal damage. Or you can do this when it has some of its side exposed.

Second/third: use a rifle or sniper to hit shots from a far, this is because the boss has a gun now. It it used at close range, this gun is a shotgun.

When you defeated the boss it will drop a shotgun with 20 ammo, this CAN NOT be reloaded and only to be used when needed. Also you will unlock a new load out item or anything so you can start with a new item or gun.

Thank you for reading this.