: How do I get into the area behind the south blast door? The last few enemies
: are in there, unless you count the civie S'pht.
If you mean the southernmost door on this map (I think I was thinking of the westernmost door before), then there's no enemies you're supposed to kill in there; and as you can see, there's not really anything there, it doesn't even have a floor. There might be some to circumvent the "it's ok if you leave 8 (or however many it is) enemies alive" feature of Extermination missions, but they're not meant to be killed.
(Does that number vary with difficulty level? Might that be causing a problem here? If so, it should cause a problem on levels like Hysterical Womb too, though).
You shouldn't have to kill the civvie S'pht either, but it probably wouldn't hurt to do so anyway, just to be safe.