This trophy is titled "You Never Forget Your First" and is for breaking the opponents barrier for the first time. This simply means that they keep attempting to block your attacks and after a while you break their guard. Heavy attacks are a good way of doing this, naturally.
If playing a second controller, simply hold back on P2 and unleash hell with P1 and it'll happen eventually. Personally, I won this against the AI who was blocking the whole time at random. This can also be done online with a friend, making it much easier.