Sebillian guard aliens appear to be insanely tough. It took me like 10 rifle shots in close range(2-5 tiles) and 1 direct grenade explosive to kill one of them and 3 others took 3-4 grenades each.
It appears that you've forgotten to remove some alien materials from the storeroom and there are also some researches available when there should be only one!
http://i.imgur.com/I0RzDjO.png
http://i.imgur.com/wJs5slQ.jpg