Place your soldiers to the west entrance of the UFO, make sure no soldier is actually beside the door. Now use one of your soldiers to enter the UFO with run mode on. Kill the alien in the first room (if there is an alien), then use another soldier with run mode on to proceed through the next door. If not, use the same first soldier to proceed through the next door. You will find yourself in a corridor, search a few steps through this corrider for aliens and kill any aliens which appear. Other aliens will come on their turn. Kill them also until no more aliens will come.