So. You have a being molded from the consciousness of two of the most powerful mutants on the planet (Magneto and Professor Xavier). Now, we do not know enough about Ragnos to argue his proficiency. I am not aware of where it states that Ragnos is capable of destroying solar systems. Judging from your response, it was merely an assumption. I think Illustrious may know, so we'll wait for him to show up. If it's true, then perhaps he might beat Onslaught.
Onslaught possesses Xavier's power - and has the willingness to use it. Force or no Force - I highly doubt NJO Luke or Ragnos are immune to the powers of the most powerful telepathic mutant on the planet. Then throw in Magneto, who controls magnetism - as well as the ability to control the Earth's magnetic field - you have two dangerously powerful mutants combined into a ruthless monster.
He can simply cause Ragnos and NJO Luke to forget who the hell they are, place them in an illusion, and kill them. We KNOW precisely what Onslaught is capable of.
We do NOT - on the other hand - about Ragnos.