Kill all three Alien Rulers, I found that the rulers only react to units in it's LOS so you can get a bunch of damage onto it with Overwatch shots from soldier that were out of LOS at the time of setting to OW, but be aware it will react to OW if your unit is in LOS when setting OW.
Also, another thing is that they don't move on the standard alien turn, so if they are the only enemy in range, ending your turn early to prevent reactions sometimes is handy.