Onslaught is a first-person shooter that centers around the Elite Interstellar Strike Force. The player is a member of the team sent to a research colony after contact was lost. Upon arrival the team is immediately attacked and all alien lifeforms need to be eliminated. The player needs to complete 13 levels grouped into different tiers and to progress all levels in a tier need to be completed, including a boss. In addition there are three difficulty levels. The game has the player move through the environment to reach another point, taking out objectives, or surviving for a allocated amount of time. The different weapons include guns and a laser whip. The game includes support for cooperative play, for up to four players with the Wi-Fi connection. Competitive deathmatch games are not supported.
The Nunchuk is used to move and the pointer function of the Wii Remote is used to aim. Motion controls are also supported by shaking to reload, throwing grenades and unleashing the whip. Insect blood can blind and damage the character and it needs to be wiped off with a swipe. When playing alone the team mates are controlled by the computer AI and they can be issued basic commands. Later in the game it is possible to drive a vehicle.