Onslaught

Onslaught is a type of mission where you have to eliminate all waves of zombies. It is the only mode in earlier versions of SAS: Zombie Assault series.

SAS: Zombie Assault 3
In SAS3, it is the only mode available in solo play. You'll have to survive against 3~8 waves of zombies in a map. The player(s) will be awarded some health after each wave.

SAS: Zombie Assault 4
In SAS4, it is the first mission available and also the mission the new player starts in.

Plot
A dropship in an infested space station cannot depart because its door ramps are stuck. The player(s) must survive and kill all zombies for at least 6 1/2 minutes.

Layout
It's a dropship with door ramp on the right, a large room with boxes in the middle, and side passageways on top and bottom of the central room. It's one of the smallest map, so becare of being swarmed or cornered by zombies. However, in terms of boss fighting area, it is actually larger, with more maneuvourable obstacles to block the boss' movement compared to Survivors

Loots

 * At the 1:30 mark (or 2:30 mark on level 2+), the pilot will drop 2 loot drops on each passageway of the map, just outside the central room
 * After At around the :40 minute mark (1:00 mark on level 2+), the pilot will open door to the forward jump seats where there's a loot drop and a medkit
 * Between the 2 barricades the player must defend, there's a black area which can be opened by shooting at it from the right, which will reveal a loot drop and a medkit.

Singleplayer
In start of the game, try double-blocking the both entrances with boxes because it's most likely that the barricades will fall. When the boss comes, fall back to the center area and move around the boxes, making the boss chase you in a circle. If there are too many weaker zombies, shoot the explosive barrels to kill them. Fall back to the foward jump seats if you need to. If the tall, darker boxes in the center room ever get in the way of you shooting the boss, you can shoot them to destroy them, since they'll just get in the way.

Multiplayer
Unlike singleplayer, where you work alone to strategize, you will have to cooperate with multiple people to achieve victory.

Do not double-block the ramp doors with boxes. You can use all that box-pushing time to help shoot zombies and not have to worry about the ramp doors. Also, since you're pushing boxes from the inside of the dropship, you are making the area you're shooting in much more open, making escapes from strong zombies or bosses more difficult when they destroy all those lined up boxes in a matter of seconds. Lastly, when you double-block the ramp doors with boxes, you are shooting zombies from further away from the ramp doors. This makes your weapons' "damage dropoff" point much closer to the ramp doors, meaning that your weapons will deal less damage to zombies, since they're further away. Forcing you and your teammates further away from incoming zombies also increases the spread of any shotguns you or your teammates happen to have, decreasing the accuracy and reliability of those weapons. Double-blocking the ramp doors is cumbersome, and in the end, only hinders your team in shooting zombies. To conclude, do not push boxes around on Onslaught.

Communication is key to victory. At the 2:30 mark when two loot drops are available, be sure to tell your teammates when you're going back to pick up that loot. The same goes for the medkit and loot drop available at the 1:00 mark. Before the game even starts, you can propose ideas to your multiplayer on what to do, such as collecting the loot drops after the boss is killed.

When either of the ramp doors fall, do not rush to the center stash between the ramp doors alone. Have your team coordinate when to break into that stash as a single player will most likely get overrun by the stepped up zombies.

Triva
Notice that the whole map looks like the evac dropship from mission Vaccine.