The one thing that makes hit stick hits online easy for some people is because their opponents tend to get predictable in the way they run. They'll always bounce outside or they'll always try to cut a certain direction in open field and so it makes the timing really easy. A lot of online players try to big hit every tackle and you can usually just make quick changes of directions and defeat that tactic.
When you're the one trying to big hit, it is 100% timing. There is a lot less leeway in NCAA 13 than there has been in the past. Too early or too late and you'll bounce right off the ball carrier or flat miss. I wish I could help describe the timing but it has always been just a feel thing for me. Just keep practicing and pick your spots, start by big hitting when you know you've got help from CPU controlled players in case you miss then as you get a feel for it, you can work on open field big hits.