A quick fix here is to change the enable_push and event_push parts of the system to enable_entity and event_entity, then making sure the newton object has a push value of 0 and is not passable. Then the player should react much better to the boxes. You could also increase the mass and speed values of the impulse like done with weapons in the event_entity (formerly push) IF-clause, right below the event code for being shot.

Reading the documentation and going through the samples will help you a lot and is really neccesary to know how to do all of that (and more).

-Rhuarc