I thought I'd never do this but I picked Mega Man and just abused his Heavy Beam shot. I also put it on super easy and maximum damage. I just kept spamming his shot until I had 3 super meters and would then just unload and would eventually connect.
Against Onslaught Mega is short enough to stay low against his Beam and is able to charge that shot while blocking, as soon as he was done attacking I'd let loose and recharge. After he goes into 2nd form i'd wait for his first attack and tag in Shadow Lady to regain Megas health. Unload all 3 meters with her laser beam and then just jump kick him in the face, if he isnt already dead. Once Shadow Lady dies Mega should have a meter and just use Hyper Mega.
I used this once got to him and died when he had a pixel left, then I just beat it t
First off, I used Ryu & Chun-Li, I made it to onslaught and if he was about to kill me, I'd hit start or have someone hit start on the 2nd controller, this way I could be saved without loosing a round. The funny thing is, I actually lost, I still hit start on the 2nd controller then I hit start on my controller, we both picked our characters, I beat the 2nd controller and then went on to beat Onslaught and STILL got the chieve!