"Destroy 50 objects or characters using the scream and super scream ability."
Do you have to use both scream abilities 50 times, I used the super scream and that kills characters and I've used it over 50 or so and still haven't got the achevement. Normal scream only seems to work on glass.
If your wanting to go through and get it, the parcel is at the end of the level. Once in the last room you go all the way to the right you will need a mechanic and a scholar you then take it all the way to the left side of the room and drop down onto a little area, you will need to destroy a pillar and then build the postal box. And if you haven't already grabbed it pull out a mechanic and fix the broken lego thing next to it and it will bring up an area that gives you an artifact