The point of Onslaught is to connect the dots... all the way to the enemy's base, and then blow it up.
You start with your own base, and it will "power" any number of adjacent nodes. To claim a node you must walk through it. (Being in a vehicle isn't good enough.) If you can't seem to claim a node, its likely not to be linked to your base (or another node you've clamed and built).
Nodes will slowly transition by themselves from "starting" to "done" but you can hurry them along with your (green) link gun's alt-fire. While you can link with other players to speed this up, that strategy seems no faster than all of you linking directly to the fledgling node. Still, two link guns are better than one!
The key to Onslaught is SPEED.
Usually the team that gets there first wins.
If possible Mantas should team up and hop as quickly as possible along a direct (linkwise) route to the enemy base. Building new nodes as needed, and stocking up on ammo along the way. If you're careful you can carry other people on your Manta's "wings."
Often just owning more nodes sooner will mean having more vehicles, weapons, and better positions on the map. So start soon and move fast!
If spawn too late to claim a manta or Raptor, try using the Hell-bender, but fill it up first! More people means faster built nodes. But don't wait too long. Try not to go anywhere on foot. Its too slow! Even if you can't get in, you can jump on top of most vehicles and just ride along. Although sometimes you can be killed this way quite easily.
If you can't find or ride a vehicle, at least make sure to get an AVRiL, as you're almost certainly going to face enemy vehicles, and you're gonna need something with punch! It may take some time to figure out where to find them, but its worth it!
You can only attack an enemy node if you own and adjacent (link-wise) node. Otherise you'll find a force field protects the node from your assault.
Any weapon is good for attacking an enemy node. Goliath (tank) main guns are particularly effective. But try to save your link-gun ammo for building nodes or reparing vehicles. If you can team up with vehicles, especially doubling up with Goliaths or Mantas or Raptors, you can muster a lot of fire power and/or speed, and make short work of enemy nodes.
Don't forget that you can "use" active nodes to teleport to other active nodes. This can be helpful if you want to go looking for fresh vehicles.
The mine-layer can be extremely useful in defending against attacks on your nodes. Don't leave home without it!