If Halo 3: ODST's campaign is "Combat Distilled," then its co-op multiplayer mode, Firefight, is the product of the same process, further refined. Firefight is combat distilled, then filtered; then distilled again and again. While the story mode can already be played cooperatively with up to four players (with or without scoring enabled), Firefight re-purposes eight of the sets from the campaign missions into closed, gladiatorial arenas where the odds are heavily stacked in the Covenants' favor. It's not a complicated formula. We've seen Gears of War 2 similarly extract itself into Horde Mode, as well as a number of other recent shooters spinning off their core gameplay elements into various arcade-like challenges.In all, Firefight contains ten maps (two are featured twice as night and day versions with slightly varied ingredients), and three of the ten are unlocked through campaign progression; though anyone can play on a locked map, so long as the match host has unlocked it. Likewise, except for the Rookie, character skins are unlocked through campaign play. (Sgt. Johnson will be a bonus skin for ODST pre-orderers.) Firefight lives on the campaign disc (separate from "vanilla" Halo 3 multiplayer; contained on a second disc), and players are bound to the limitations of an Orbital Drop Shock Trooper: non-rechargeable health, no dual-wielding, and so on. You're no Spartan.