I hate flat hitters, they are usually tough opponents. You have to be active, for example try to always give topspin to make the ball lower and more difficult to control. Counter-hitting is difficult. Simple blocks are often not effective against this style.
I used to be a flat hitter in my early TT years, and quite good at it. i played with Tibhar 4L and Mark V. Short pimples on FH (!!) is a good choice as well. Penholders hitters with short pimples are really annoying.
With my current equipment, i should not flat hit as much as i do now, because its easy to make a mistake, but i have some good remnants of my former style. Against a chopper, for example, if the ball is high but with backspin, normally the best approach 80% of times is to topspin it back calmly, but more often than not i will try to kill the ball.
What is important when i flat hit the ball, is that I open my wrist (thats the main little secret), take a good backswing, and if I'm not very well placed, i'll always smash straight, the error rate is less than trying to go for one side