what did you mess around with? maybe post a screenshot of your particle setup…
I´d go with mparticles, I can imagine though, that the leaves won´t fly off nicely, if you just let something fall in them, so you might want to time a wind force to blow out at the moment of impact.
You´d need to set up the flow to first make a pile and then after it formed a pile, time your collision object to fall in (you could key it by hand or just run a simple MassFX sim without the pile of leaves).
Either that or have the collision object as a birth group in another event and wire that into the same Pflow and time it to fall into the leaves, after the other event has made them fall into a pile.