How do you unlock onslaught in transformers war for cybertron decepticons?

Onslaught is not a playable character in *Transformers: War for Cybertron: Decepticons*. He is only a boss character you fight in the game.

If you are referring to unlocking Onslaught in the sequel, *Transformers: Fall of Cybertron*, then you need to:

* Complete the "A Bridge Too Far" mission. This is the mission where you first encounter Onslaught.

* Defeat Onslaught in the "A Bridge Too Far" mission. This will unlock him as a playable character in the game's multiplayer mode.

Keep in mind that Onslaught's specific availability and unlock requirements may vary depending on the version of the game you are playing (i.e. original release vs. remastered version).