There are some nice tutorials for ut2k4's onslaught. The basic idea is the same like in Nexuiz' onslaught. Of course you need to ignore all ut2k4-specific-stuff that gets mentioned in these tutorials (and that's a lot of stuff like vehicles. the link gun, the mini-map etc.).
Really cut it down to the basics, and you have Nexuiz' onslaught:
Capture the nodes.
Get a "link" via the nodes from your base to the enemy's base.
Attack the enemy's base.
ut2k4:
http://www.youtube.com/watch?v=mGIY5sL0bUg