The Watson HE99-X "Firefly" is the Shai-Gen Rocket Launcher. The Firefly holds 5 rockets at a time with 15 in reserve. The Firefly has a homing function but no zoom function. Usually one person in Shai-Gen group will use the Firefly. If the agent has a high agility level then he can take down patrols of Shai-Gen without being fired at and by using as little ammo as possible. Statistically, this is the best explosive device in the game, and is a popular choice amongst high ranking agents. Shai-Gen often use this weapon en-masse, and can quickly obliterate even the most developed of Agents. Shai-Gen often use it in strongholds, on roof tops.
The best way to face a gang member with one is to keep moving and try to jump as much as possible (due to their homing capabilities), it is advised to use an accurate weapon to dispose of the gang member quickly because you'll be moving around to dodge fire.