I just stuck with Ryu and Hulk. On the first round, I crouch with Ryu while blocking attacks, when Onslaught gets close, jab him in-between attacks, and during attacks that can't reach you. On the 2nd round, I just keep hitting his head with Hyper Combos in the air (Shinkuu Hadouken) and Hulk's fist attacks. It helped me win the no-round-loss achievement.
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