The Grim Adventures of Billy & Mandy is a 3D arena-based fighting game, similar to other games such as the Power Stone series. In the game, up to four players battle in a three dimensional arena and perform light and heavy style attacks in an attempt to deplete their opponents' health and knock them out.[1] Each of the eight levels contains elements that can affect the battle, such as enemies that will attack the players or traps that can be triggered.[1] Most levels include multiple sub-arenas and will transition from one to the next after a short amount of time has passed, such as rising lava in an Underworld cavern forcing players to ride a giant snake to escape.[1] Players can pick up and wield different weapons that appear, such as a halberd or a club, to do additional damage.[1] Treasure chests will spawn that contain items, including weapons, healing items, and Mojo Balls that fill the player's Mojo Meter, which can be filled up to two levels at a time.[1] When the Mojo Meter fills to the first level, a Mojo Smackdown can be performed where the character attacks an opponent with a barrage of attacks that will knock them out if it connects; if the opponent's meter is also filled, they can perform a Mojo Counterattack to interrupt the Smackdown and deplete both characters' meters. When the Mojo Meter is filled to the second level, a Mojo Meltdown can be performed, instantly knocking out all opponents.[1] Players have a limited number of lives in each battle, losing one each time their health is depleted; when a player runs out of both lives and health, they are temporarily stunned, at which point an opponent can attack them and trigger a quick time event sequence called a "finishing blow", which will eliminate them from the battle. If the finishing blow sequence is failed, the player will escape and resume battling, though they remain vulnerable to other finishing blow attempts.[2][3]
Several gameplay modes are available. In Story Mode, players must complete five battles against a series of computer-controlled opponents, followed by a final battle with the brain-eating meteor from the episode "Little Rock of Horrors". Story Mode supports both single player and cooperative two-player play. Vs. Mode allows up to four players to battle against one another, with the option to play different types of matches and customize the game's rules.[1] Some battle types include "Bask-Eye-Ball", in which players must place an eyeball into a goal; "Crush the Horde" a battle against several waves of computer-controlled enemies; and "Skull Keeper", a Keep Away-based mode in which players attempt to hold Thromnambular's Skull for as long as possible.[2] Mission Mode features 45 missions to complete, in which players must achieve specific objectives using predetermined characters in set scenarios.[1][3] Additional characters, stages, costumes, and rule sets can be unlocked by completing missions, finishing Story Mode with each character, and fulfilling other objectives.[3] An "Extras" menu features character profiles, information on unlocked rules and items, a gallery of unlockable concept art, and trailers for other Cartoon Network products and Warner Bros. Pictures films.
The Wii version of the game incorporates some motion controls using the Wii Remote and Nunchuk: players can perform heavy attacks by swinging the Wii Remote, as well as using its infrared pointer to point at a series of onscreen targets to execute finishing blows. Traditional controls are also supported via the GameCube controller.[2][3]
