well... you need to have a physical response that has the "move event actor only" box, set the multiplier up to about 1.2 or whatever, and adjust the weights so that the actor that hits it is "heavier" than the actor getting hit...
IF you want it to hit then slow down eventually, then put a physical response on the ground floor so that the collide actor (the one that is going to be hit) gets slowed down, then make the final multiplyer below but more than 0.001 (that will stop it dead in its tracks...) also make sure that the box for move collide actor only box is chosen!
Hope this awfully long boring speech helps