From The TimeSplitters Wiki

The Grenade Launcher

Another typical weapon that shoots grenades, but theres a catch, its specialty is that it can shoot balls of flames at your enemy. This weapon was seen the Aztec Temple and it was use to fight the Stone Golem. Scine the Golem is tough, Captain Ash had to use the Grenade Laucher to at least damage the beast.

Elite Yoshi's Tips

Use the flame for this gun to put others on fire, or, put flames on the all doors leading outside the room, that way, your enemies can't leave.

To find more tips on this weapon or other weapons, visit the Weapon tips page.