Overwatch has a -20 aim penalty IIRC, so generally it is not very efficient in taking aliens down with overwatch. Especially for Rookies and Squaddies they are better to hunker down rather than use overwatch.
With EW I always have a soldier with mimetic skin to be the scout and find aliens, so the scout will have zero chance to be shot at. (In vanilla game, a support with sprinting and skeleton/ghost armour will be the scout.). The rest of the team will be slightly behind in cover, ready to pounce. The snipers will be safe even further back (but not too far in case some aliens magically teleport near them).
Once aliens are found, then the squadsight snipers will shoot first, while the rest of the team advance to help out.
If, after the snipers have soften the aliens up, I estimate that the team STILL can’t take out revealed aliens after a turn, I retreat as far back as possible so aliens can’t even grenade my team. On impossible, mutons and elites have a nasty tendency to grenade even a single soldier, and they can toss a grenade so much further than a normal soldier. Better safe than sorry. After a turn, the scout with mimetic skin will still have LOS of aliens and the snipers can continue, while the rest of the team will mop up.
If the aliens retreat, then the scout will advance even further to look for targets for the snipers.
Then rinse and repeat.
Team composition will be:
1 In The Zone sniper, 1 Double Tap sniper. Both take Squadsight (goes without saying), Disabling Shot (you don’t need battle scanner if you have a mimetic skin scout, and you need Disabling Shot when you are fighting Sectopod because sometimes you can’t take them out in 1 turn), Opportunist, and Damn Good Ground (gunslinger is practically useless because snipers don’t get much better with CQB even with gunslinger.). Double tap is sometimes more useful than In The Zone because sometimes you really have to take out a Muton Elite without assist, and Double Tap sniper is the only one who can do it (Headshot + a normal shot).
1 Assault with mimetic skin. Take all the damage dealing skills. Lightning Reflex is a MUST. But don’t take Close Quarter Specialist because it will ruin the scouting ability. Assault is here mostly for the Lightning Reflex to get the team out of a jam, and Run and Gun is really good at mopping up that one last tough alien behind cover.
2 Heavies. Both pick Bullet Swarm (holo targeting isn’t worth it unless you take Suppression all the way to Mayhem, but you are still sacrificing too much because you have to give up shredder rocket.) Both pick HEAT ammo (goes without saying), Shredder Rocket (for sectopod or a mob of six+), grenadier and rocketeer. Use medal/gene mod to improve their accuracy (MUST). Against late game aliens they are just as deadly as the snipers.
1 support with all health related skills, sprinter and dense smoke. This is necessary for impossible and/or ironman. Otherwise any class is fine.
My In The Zone sniper has the highest mission to kill ratio, average about >3 kills per mission, all thanks to this strategy. Once a group has been soften up and exposed, the In The Zone sniper will just mow them down one by one. It is the ONLY way for me to finish terror mission, late game in impossible, with all excellent rating. Fighting 4 sectopods in one single terror mission is NOT fun, but I did get a “hell yeah” afterwards with no casualty.
This strategy also got me through the “Army of Four” achievement on classic. Take one of each class, and make the support damage dealing rather than a healer, and get through the story missions ASAP.