Kill it with fire
From Halopedia, the Halo wiki
The Kill it with fire medal is awarded in the Halo: Spartan Assault Flood missions for killing 50 enemies with fire without being hit. It is represented by a stylized blue shield covered in flames. The napalm grenade is the only incendiary weapon in the game.
The medal's name is a reference to an internet meme of the same name.