Tactics (EU2012)

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Pod System

Aliens now come in groups of 1 to 3. Some of these groups may patrol around the map by teleporting to random points around the map, but are tactically dormant (i.e. they will not open fire or take cover) until you "wake them up". Note that once awoken, aliens can act fully even when outside your visual range, including Overwatch (and in the case of Sectopods, the Cluster Bomb attack).

Similar to massive-multiplayer online game instances, "waking-up" multiple groups of enemies is just plain stupid. The best strategy is to wake-up and deal with one group at a time. When scouting for enemies, don't move too far ahead, especially with a 2-move dash. When a group is activated, it may be prudent to then back up and get into defensive positions to greet the enemy.

Enemies can be in two states: activated or not. When you start a map, all aliens are not in activated state, and are bunched up in packs of up to 3. They roam the map (actually, teleport around the map until next teleport will put them inside your LOS, thats when they make a movement animation) or even stay in place until discovered. When spotted on your turn, or coming into your LOS on their turn, a small cutscene will play, and aliens will always make a move, usually into cover. They will never take an offensive action at the turn (yours or theirs) at which you met them. Only once activated, starting from their next turn, aliens start to act as individual units, taking turns in the same manner as player does. (two actions per turn, move/move or move/shoot, overwatch, etc.)

This means that as long as you have no activated aliens on the map, you are completely 100% safe from any kind of harm. You can have every enemy carry a grenade and yet bunch up your soldiers next to each other and never be in danger, as long as those enemies are not active yet. You can be in the open and never be in any danger as long as there are no active enemies on the map. You never have to fear an enemy that is not activated. In a situation when you have no active aliens on the map, you can play a completely different game, and to a big advantage.

Also, notice that landing a shot on an alien that you have spotted but that has not spotted you (this can rarely happen when you're behind cover, or easily happen with Battle Scanner) will activate the pack. Misses that travel close to the pack will also activate them, as missed shots provide a bit of vision. Same for explosives.

Do note that, sometimes, these wandering packs may 'teleport' into the middle of your squad, and that if savescumming, unactivated packs might be in a different location (even from the same savestate).

Vision

It should always be assumed that any uncleared fog harbors more pods. This limits forward options and is the appropriate way to play (do you expect not to get punished?). The game often hovers an unactivated pod at the periphery of vision while in contact with active aliens, but this may be unnoticeable on lower difficulties. Situations where one must take a risk going forward are less often than one might think. Limiting yourself will make it all the more rewarding when you truly "have no choice".

Combat

Have a guaranteed backup plan, e.g. a Grenade or double Grenade kills, or someone has a 100% kill, if intending to take pot shots (that could go horribly wrong). 90% rockets or 95% shots are not guaranteed kills.
Suppression+Smoke Grenade+high cover is safe against Sectoids, Floaters, Mutons at range, and Sectopods, given they don't run the reaction shot.
Try to find Overwatch spots where the enemy can not Fire back from, such as a sliver of vision containing no viable cover or places where the enemy would have to yellow move to. Those are great penalty free reaction shots if the enemy crosses the visible region. Hard to come by but rewarding when discovered.
Plan before you commit. This is important for 2 reasons:
1. Aliens get a free move when they are seen and if the alien moves into view of a soldier that has already been put on Overwatch they cannot use that turn to fire. You have to rely on them having a reaction shot during the alien's turn.
2. If you want to move them somewhere else you can't because they are locked once on Overwatch.
Alternatively, put several secure units (like Snipers) into Overwatch before revealing enemies. Your soldiers will open fire on aliens as they scramble for cover.

Hit Chance Ratio (or when "low cover is NOT no cover")

A simple heuristic to evaluate a play is to add up the hit chances of XCOM and the implied hit chance of the enemies' subsequent turn to get a hit chance ratio. Higher number is better.

Assume you have four soldiers and there are only two high cover spots available to Fire at two Sectoids in high cover, the hit chance ratio (assuming first mission stats) is 2*(65-40):2*(75-40), 50:60 or ~.83:1, if only using high cover. Not great assuming there are more pods left. However, there are plenty of low cover spots lying around which would allow us to Fire with all four soldiers. That should mean the Sectoids would target the unit in low cover, 4*(65-40):2*(75-20), 100:105 or ~.95:1. Better because it allowed XCOM to increase their firepower despite using low cover (though the ratio is still not the best). Let's add one more piece of high cover, that would again change the ratio to 3*(65-40):2*(75-40), 75:60 or 1.25:1. Less total hit chance than the four soldiers in low cover, but that added piece of high cover tipped the scale again compared to the two high cover option. This is to demonstrate that the maxim of "low cover is no cover" is not always correct.

Instead of calculating the hit chance ratio, we can actually determine whether using a lower defence option will improve the ratio without requiring XCOM's hit chances (assuming the average hit chance of the XCOM squad for each soldier).

The engagement threshold t of when to consider s free soldiers using the 2nd best of two defence options is s·a/d, with a being the alien hit chance with the 2nd best option and d being the defence difference between the two options. In another way, to consider the 2nd best option, the ratio of soldiers using the 1st best to soldiers considering the 2nd best is at or worse than t-s:s.

If f is fixed soldiers using best defence option and t=f+s, f/(a-d)=(f+s)/a, fixed soldiers over worse alien hit chance vs fixed plus free soldiers over better alien hit chance, derives to s·a/d=f+s=t

Let's take the example of 75 aim Sectoids and high cover(40) vs low cover(20).

In this case, a=75-20=55, d=40-20=20, making t=s·a/d=s·55/20=2.75s. Either the ratio of soldiers using high cover to low cover must be equal to or less than 1.75s:s or the total number of soldiers is equal to or less than 2.75s. If s=1, there must be at least 1.75 (so 2 whole in this case) soldiers utilizing high cover to not expose the third in low cover. That would only lower the hit chance ratio.

However, if s=2, the total 2.75s becomes 5.5, or the ratio of high cover to low cover is 3.5:2. This means that if you have 3 or less soldiers in high cover and are considering 2 soldiers to using low cover, you should if you plan to increase the hit chance ratio and Fire.

A more extreme example is to consider when to go uncovered. Let's say it's 75 aim Sectoid again but low cover vs no cover. a=75, d=20, t=s·a/d=3.75s. If there happens to be only two or less pieces of low cover available, you can allow yourself to go rambo knowing you made the locally optimal call. If there are three pieces of low cover with a squad of four, better not expose the fourth.

This assumes that the enemy can always hit a soldier using the 2nd option. XCOM engagement can be stratified though and so dividing which set of units is engaging with the enemy's set of units is something to consider. This also isn't a confirmation that simply Firing is good play. The ratios in the initial examples are atrocious considering there are 3-4+ pods every mission. But there may come a point where "everyone has a plan until they get punched in the face". The ratio is a gauge of how hard you are getting punched and the threshold is how to correct it given no other options.

Generally, the lower a/d is and/or the more soldiers with the 1st best defence option available, the less likely that it is correct to utilize the 2nd best defence option.

Mind Wall

If you are tired of mind controlling aliens after you got Psionics and if you have lucky to have 5/6 Psionic soldiers you can set your psi squad to be permanently in a Telekinetic Field that gives all units (including the aliens) inside +40 Defense. The ability has a 4 turn cooldown so you need 5 soldiers equipped with it to always have 1 unit activating it at the end of your turn. This tactic works ideally on open areas like smaller UFO maps and most abduction and Council missions. Even if the aliens are protected within the field you just need not to set your soldiers on Overwatch so that they'll not fire at the aliens, and if you combine it with Dense Smoke and/or terrain cover you'll get +80 Defense stats. The only vulnerability of the field is to grenades, so take out first any aliens with explosive weapons.

Enemy Unknown Exploit

If AI unit has no AOE attack, has no enemy in LOS which is exposed/flanked, is himself exposed/flanked, and sees a unit which is on Overwatch, it will completely skip its turn. AI seems to run out of options and "panics". Therefore, you can "lock down" units like Sectoids and grounded Floaters, or Mutons if they already used their grenades, or Thin Men if you carry Medikits, by taking a flanking position with one of your units and putting another visible unit on Overwatch (can be the flanker, can be a different unit).

This allows you to advance your Arc Thrower carrying soldier, take free shots on the enemy, or just save yourself from harm. Using this is way superior to just taking the shot with your flanking unit because in case you do not kill it, AI will move to cover on its turn and fire back. One Overwatch will keep all flanked enemies at bay while giving yourself freedom to deal with them in whichever way you prefer.

Since the flanking unit does not necessarily have to be the one to Overwatch, dash a unit to a secure position where it has an exposed view of all enemies and, so as long as a unit has any line of sight on these same enemies (not necessarily an exposed one), Overwatch will lock them down, except in some cases mentioned above.

Equipment

Ghost Armour is the obvious choice for all classes (maybe Psi Armour for Psi), but for snipers you may want to equip with Angel Armor. Because of a bug in the game, Angel Armour grants you Height bonus and Damn Good Ground bonus even though the Angel Armor strictly says it shouldn't. With these bonuses plus a scope, your chance to hit jumps to near certainty even for aliens with full cover.