Now that I have gotten back into the swing of things with this game he's fairly easy, patience is your friend with Onslaught, and a good assist.
For the first form I usually send out my assist in the middle of his teleports to stagger him and to get a couple free hits. Another good thing to have is a character who will not get his by his beam attack when they are ducking, it can get you a good free 4 or 5 crouching attacks on him.
Second form it's all about picking your spots with a jumping high kick, and supers that can reach him. Dodge the hyper grabs and stay defensive and watch out for the eye beams. Jump over the charging hand attack and he's really not all that tough.