Depends on which Onslaught this is and what constitutes a win against him.
If it's the one Mindless Hulk smashed, and that's what you're after, then WWH would easily get the job done. If on the other hand, you want his energies absorbed and completely ended. Then it might require the whole classic Avengers and Warbound too.
Thanos is the next biggest problem, though Annihilators would be able to keep him busy until Hulk was finished with Onslaught (given the former criteria for victory).
Voidtry could be utterly neutralised by Surfer, imo.