Dude, Onslaught is basicly energy. His attacks are energy based. Monarch(Captain Atom) is able to prevent others from using their powers, which theorectically leaves Onslaught defenseless. 🙄
Also, Monarch is able to create replicas of others, so he could just make a replica of Onslaught to fight Onslaught. 😄
Even if Monarch couldn't defeat Onslaught through force, he wouldn't be able to lose, because his armor is virtually indestructable. Onslaught wouldn't be able to even hurt him. 😎