Turn your Main goal to terminate all the Sectoid as these are also a threat to your squad. You must start this at a very early stage and whenever you get the opportunities to target them. You can find their corpse at the back of the base so once you are done taking down, head directly to the research lab. Now start helping Dr. Tygan to perform Sectoid Autopsy. This is a time-consuming process so if you want you can continue with side objectives until he finishes the work.