XCOM: Enemy Within

Also see the page for XCOM: Enemy Unknown

• The most overpowered unit in the game is a Sniper with Low Profile and Mimetic Skin. Mimetic Skin makes your character cloak whenever they're in full cover until they take an offensive action or end a move action outside of full cover. Low Profile makes partial cover count as full cover. Imagine an army of snipers. Now imagine they're all invincible because the AI isn't capable of targeting them

• Shortly after you do the Alien Base raid, you'll get an event where the aliens raid your base. It'll deploy random characters from your roster. Make sure you keep a good number of soldiers trained up at least to rank 3. There's no warning for it, so if you get unlucky with the timing, your entire A-team might be injured.