Onslaught was not only a genius, but described as a "super-genius." Though when finding out about Onslaught, I have come to realize that the databases such as wikipedia are not a reliable source in finding out about Onslaught's power.
Let's be honest. There's two factors that have to be factored in. One, Onslaught isn't as strong as they say he is. Two, the writers blew and didn't know all his powers. For example, Onslaught was able to control magnetism. How come he rarely used it? But used Magneto's powers to create forcefields and "uprooted the Earth beneath them."
In my opinion, which I have had for a while, when Onslaught absorbed the powers of Nate Grey and Franklin Richards, he only absorbed a part of their powers and not all of it. Because if he did, he would of been unstoppable. Onslaught was no idiot either, he was a super-genius, he would of known immediately how to control the powers.
So what is Onslaught? He has amazing physical strength, able to fight an amazingly savaged Hulk (so savage, Hulk's eyes were red and it seemed as if his size even grew). He had the power of magnetism, and telepathy (though in the last fight, they countered that thanks to Iron Man). He also had telekinesis and some reality-warping powers. Using it, he created a sun (which Thor and Storm took care of).
The sacrifice in the end? Completely unnecessary. Doctor Doom revealed that it was energy, and it could of been absorbed. How come they didn't capture the energy, or absorb it and send it into the sun? Could of been done, but it didn't. Bad writing? Not necessarily. If you are under THAT MUCH STRESS (Earth almost was destroyed), it's hard to think of logical strategies.
Now, Galactus vs Onslaught? Onslaught will get a few good blows in, but in the end Galactus wins.