Genestealers have greater mobility and many points of entry. When you become surrounded, use Command Points and place two of units back-to-back. The Overwatch command is more effective with more range, thus this formation works best with maximum distance between you and the enemy. Kite backwards and shoot your pursuers, even though it costs double the Action Points. Try to keep as much distance between your units and Genestealers. This will allow you time to recover from weapon jams.
Easy "Exterminator" achievement
Position your autocanon in the middle lane in the south-most room with a normal Terminator guarding the lane to the right. When a lot of Genestealers appear, place both persons on Overwatch. Save some AP for unjamming.
Easy "Red Thirst" achievement
Play mission 2, "Exterminate" with Gideon. Place him south by himself in defense mode. He will rack up kills quickly, especially if there are more than three Genestealers within six tiles.
Easy "Unbowed and Unbroken" achievement
Move from room to room with every marine in one group. Cover the exits, resurrect the marines, and make sure no one. You do not have to complete the mission. "Unbowed and Unbroken" will be earned when you resurrect the last marine.
Special units
Although Librarians and Sergeants are not powerful units on their own, they help the rest of your units with free shots and extra Command Points. Thus, they should be protected and kept alive for everyone's benefit.
Defending...