Ok I am confused by this thread by now. Not knowing how the OP plays, in general, when I played Jpen, I tended to position myself close to the table because, well, I could only block on my backhand side. My forehand loop was positioned to start a rally with slow spinny loop. After that, I would try to finish point with more loop drives after that.
When I played Jpen, if the other player smashed (and assuming I am close to the table due to my style of play), the only choice I had was quick reflex. So either you have the reflex or not. That's why the other posters say, more practice. Without quick reflex, I am not sure what answer you are looking for.
As for other suggestions,
1) once you are forced into a defensive position by the opponent, then you could take a couple steps back or some to buy yourself more reaction time.
2) Is the other player smashing flat or with topspin or with backspin? Most smashes from SP are flat so you have to open the angle up a bit. Most smashes from inverted has some top spins. If the ball is high and close to the net, the person might chop smash the ball so it has lots of backspin.
3) Are you looking for counter-smashing? If so, I still would recommend fishing and blocking as opposed to counter smashing because, well, there is no time!
A smash is meant to finish the point. It is a high risk, high reward play. Once the other player decides to smash, there are only three scenario's: they hit it into the net, they hit onto the table, or they hit it off the end of the table. If it is scenario 2, then you just gotta rely on your reflexes and your reaction time. If you want to counter smash back, I would not recommend that because if your opponent is of any decent caliber, his smash is going to end the point and there is no time for you to counter smash back.