The Fantasma is a Supremo backpack that lobs three canisters of Viviro gas onto the battlefield. Enemies in the red cloud go berserk and attack each other while the poison eats their health.
Fire it into a packed base or a group behind cover and let them tear each other apart. It clears rooms and courtyards fast. Aim it yourself, since the canisters will not find targets on their own.
Complete the story mission Libertad Rises, then buy it from Juan with Depleted Uranium.
El Besito and Poison Grenades spread the same kind of poison. Gear that boosts damage to poisoned enemies makes the cleanup quicker.