Most online ermacs use a lot of the same tactics online:
d3 into overhead: ermacs love to d3, d3, then go into their overhead starter, usually followed by a soul ball or tele. Block and punish accordingly.
His d3 is minus 6 on block, so after you block one, use your own d3 if they like to do repeat pokes. Online this can be harder then it should be (thanks NRS), but if you can get a handle on it, it really shuts down online ermac bs.
Hover Ground Slam/Soul Ball/ Teleport Mixups
This really isn't a mixup, but lots of ermacs will do the ground slam while you are full screen, then when you try to jump in anticipation of the next one, do an air or ground soul ball that will hit you for full combo. Once again, online makes this much harder then it actually is, but you just need to be patient, and walk in until you are in jump in punch range, if they take to the air you can either jump and hit them in the air, or wait until they start to come down with the ground slam, then jump punish, or bait and block the soul ball/tele.
One of Ermacs biggest weaknesses is his negativity, and one of cassies strengths is her reversal flipkick. Go into training and find the strings you are having trouble with and look at how negative they are. 242 and reversal flip kick are your friends.
Ermacs mid hitting string that has 3 parts that so many ermacs like to use in neutral (I think its f21d2, but its neutral on block) actually has a gap between the second and 3rd hit just like cassise b124 that can be armored through with Ex flipkick, make sure to punish them for throwing that string out to make them think twice about it.
Wake up EX tele can be unpunished with a NJ 2 punch I'm pretty sure, its also full combo punishable, or just play spec ops and make them hold that in the corner with your missle setups
