Well hellfire trap is specifically worded for one creature...While force wave says "All creatures" I would rule you move creatures one at a time for this type of situation, but force wave makes it seem as if all creatures would be have to be moved all at once. (I do have a problem where I take text and rules to the literal extreme, even if it makes less sense, I dont know if anyone outside of MtG will understand my pain.)
I would say the best solution would be to treat it like a zone attack were you do it one at a time like how sausageman suggested.