A bit ASCII graphics
this is the back of harvesters:
_______l__ _l_ __l__________
\ l l /
**** \_l l_/ ****
If you approach from the bow of harvester, soldiers at positions marked * are usually safe, as long as none of your soldiers go past that line, probably due to AI bug. Once your soldiers are in position, you can storm the aliens that are near enterance.
Another possibility is reactionfire baits. Several soldiers with laser weapons and medikits take position at significat distance from enterance, must have visual on it. They will draw fire of aliens, but distance and medikits should prevent any casualities, and lasers could score a hit or two with RF. Your assault team will use that time to safely get to position for assault.
If you suspect there are aliens hidden right behind the doors, fire rocket launcher and grenades through the doors into the wall, splash damage will kill them, or at least injure them, making them go out raging or duck and freeze in fear.
EDIT: damn, ascii art looks a bit different in edit mode but well...it should work.