While it sounds straightforward, recognize that melee forces will try to attack you to your face no matter how ridiculous it is. Stun troopers who should stay under cover will try to attack your soldiers up close, even if it puts them in a horrible position. Be prepared for this. Also, remember that many melee units can attack on the same turn as when they first see you. Don't be surprised when an enemy appears and instantly manages to land a blow; this is why you should consistently overwatch, especially if there are signs that kind of enemy is around (for example, Faceless on missions where you are defending civilians).