Thats not an easy exersize for any length of time,aside from experience it would be tough on anyone shorter in range or shorter in height and worse if you were both.

Coming over the outside of their jab with a jab elbow down is the block and counter, but thats a tough job to keep it up repetitively even if you are the larger and do out reach them.

Maybe your right if the other guys is more experienced or larger wait for him to move and disrupt his jab then move in.

I would be blocking it with a glove and fireing around the reverse side to his reaction. Even blocking it coming in with my elbow at his wrist or forearm distance and then being in the loaded position fireing it as I move into my range.

I think to fire low and immediatley fire high and the reverse is the key to getttig past their inital reaction cause if they are hit low or even see it go low their arm will come low in reaction so the opening is obviously instantly going to be high and visa versa. Triple up too and go for what you know will be their last reaction to number 2 shot.