Gameplay is quite unique in Onslaught, compared to most First Person Shooters. In a standard FPS game, players would be required to move from one place to another, while encountering gunfights along the way, in order to advance the story. In Onslaught, the primary objective is not to get from A to B, but to obtain the most points by completing the assigned mission at hand. Missions range from many things, they may be to hold back several waves of insect cyborgs from destroying your base or passing a certain point on the map, they might be to destroy all of the enemy hives in an underground cavern, or to reach your teammates before the time limit runs out to protect them from incoming enemies. If a player fails the mission by either dying or failing the unique objective, they are forced to begin the entire mission over again, with no checkpoints.
Each mission has it's own unique objective, after completing each, the game assesses the player based on how many kills they obtained, how long they took to complete the mission or how much time is remaining, how much life they had at the end of the mission and other elements, with the final score receiving a letter rank, with A being the highest possible. There are 13 missions in total in the game, broken up into several tiers. Each tier has 3 regular missions and a boss mission, with two boss missions at the end of the game. The 3 regular missions are playable in any order, however all 3 must be completed before the player can attempt the boss mission.
Onslaught also has compatibility with Nintendo WFC, enabling another two modes of play as well as the story mode. The first is "Free Battle", in which up to 4 players will team up to attempt a mission from the regular story mode, however while all of the players are working together, the focus still remains on the high score, as the game keeps tally during each mission and totals everyone's scores at the end of the mission. In Free Battle, it is not impossible to shoot your teammates, but each time you do, rather than damage them, you take damage yourself, however you cannot die completely, if you lose all of your health, instead of restarting the mission again, you are re spawned with your score drastically reduced, ready to continue fighting.
The second online mode is "Ranking Battle", in which the 2 to 4 players are placed in a level with swarms of various insect cyborgs, along with a boss towards the center of the level. The players are now against each other and must obtain as many points as possible by whatever method they see fit. The players still cannot shoot one another, though, the same punishment applies to those who do, the same is true for if the player dies. The round ends after a set period of time, with the player with the highest score claiming victory.
Controls in Onslaught are, much like many Wii controls, easy to grasp. The pointer and B button aim and shoot, naturally, while the A button co-ordinates your NPC allies to three different positions, whether you wish to focus fire-power in front of you, cover a wide area or keep watch on any enemies coming up from behind you. Waving the Wii Remote reloads the current gun in your hands, during which time you are unable to perform any other action, while waving the Nunchuk will wipe off any bug blood that gets splattered on your visor by close range insect cyborgs. The C and Z buttons prepare the two sidearm weapons, with another swing of the Nunchuk to use them.
There are only 4 main weapons in Onslaught, along with two sidearms and one vehicle that is available in some missions. The first is the assault rifle, which does adequate damage, ranges fairly far and fires in short bursts, it is used mostly against tougher enemies that have small weak points. The second is the minigun, which has a much higher bullet capacity, firing speed and reload, however only does about half of the damage of the assault rifle, used primarily for fire fights against enemies with larger weak points.
The third gun is the shotgun, which carries a small load and is best in small ranges with enemies that move around erratically, as opposed to stationary enemies. The last main weapon is the rocket launcher, which can only carry six shots at a time and reloads very slowly, however it has the highest damage of all guns, causes splash damage, works at a very long range and can lock on to all of it's targets. Each gun also has 10 reload cartridges, which refill all ammo in the gun and can be found around the levels at various points. The two sidearms are the grenades, of which the player has 10, and the beam whip, which can cut through normal enemies in one or two swings, but can run out of energy fairly fast, however it does recharge while it is out of use.
The vehicle is considered to be the "ultimate weapon", it is the "BKD-G9", a tank-like jeep, mounted with 2 miniguns, not only can it run over the bugs and kill them, albeit doing damage to the vehicle itself, it can tear through even the toughest insect cyborgs in a matter of seconds, these are usually only available in missions that will require the player to use it just to survive the mission, to get to a location in time, or to push through a large group of bugs that would be other wise impossible to break through.
The downside of the BKD-G9 is that it has limited ammo, depending on the mission it will have between 1 and 3 thousand shots, that can't be refilled, after which the weapon can be set to run into the bugs as a last ditch attack, after a few seconds of running on it's own, the vehicle explodes and takes out a large area of the insect cyborgs, particularly useful in missions that require the player to defend something.