Impossible Difficulty (EU2012)

Two shot hypothesis

The simple difference is this, an Assault Rifle's damage ignoring crits has a 66% chance of lethal against a Classic Sectoid if it lands. On Impossible, it's 33% chance of lethal, 66% chance of requiring two shots. There is a similar correlation, Squaddie weapons do 3-5 damage and against Classic 4HP Floaters and Thin Men, 66% for lethal. On Impossible, it's 100% of requiring two shots, ignoring crits. Further onto Mutons, Beam Weapons again require at least two shots ignoring Crits but is the same in Classic. Why does Impossible look more balanced and the difficulty curve is more steady? It is possibly the intended difficulty. In comparison, Classic looks strange as to why there is a sudden jump from mostly one shot aliens in the first two months to two shot Mutons in the third month, though they are provided more resources, due to it being Classic difficulty, to handle that spike. (Not to mention that in Classic, soldier HP is often outside of non Crit damage range, making Impossible the only difficulty where XCOM is actually behind)

The following is the chance of lethal considering 1-4 65 Aim shots at a full HP Classic(3HP) vs Impossible(4HP) Sectoid, that are all either Flanked, at low/high Cover, or Overwatch shots. Included in grey are Sectoid and Mind Merged Sectoid shots at a 4HP soldier.

Chance of lethal based on shot type and number of shots taken

Shot type	Flanked(+50 Crit)						Low Cover(20 Defence)						High Cover(40 Defence)						Overwatch(.7 penalty, 0 Crit)
# of shots	C		I		S	M	C		I		S	M	C		I		S	M	C		I		S	M
1	56.33%		47.67%		55%	67.5%	31.5%		18%		22%	31.17%	17.5%		10%		14%	19.83%	30.33%		15.17%		17.5%
2	81.68%		75.62%		83.75%	90%	54.9%		40.05%		50.05%	58.3%	32.5%		21.25%		30.45%	38.03%	53.77%		37.23%		42.44%
3	92.53%		89.34%		94.89%	97.03%	71.11%		58.86%		70.84%	76.41%	45.16%		32.5%		45.92%	53.31%	70.30%		56.78%		65.59%
4	97.01%		95.53%		98.36%	99.14%	81.87%		72.88%		83.87%	87.21%	55.70%		43.05%		59.08%	65.49%	81.36%		71.54%		79.91%

Some observations:

• For the first mission, Overwatch shots and shots at Low Cover are practically the same. If you've ever thought one or the other was somewhat good, here's the surprising news.
• There is a much higher lethality chance in Classic but another way to look at it is this: there is about a 10-15% chance difference between Classic and Impossible for shooting at high/low cover and Overwatching. This is the hard data that what may have worked in Classic will give the aliens a 10-15% higher chance of living and firing back. Shooting at cover or gambling on an Overwatch trap are all bad plays. From Sectoids on, the aliens only get tankier.
• The amount of reserve shots for a particular play will dictate whether it's a good play or not. Impossible roughly requires one more shot than Classic to have even a similar lethality chance within the given chart.
• Instead of chance for lethal, consider the reverse: all but the four shots at a flanked 4HP Sectoid have less than a 90% chance of success, meaning there is a 10-85% chance, depending on the play, for the Sectoid to survive another turn. This is not taking into account all other aliens and the amount of shots they will fire in return. Consider the concept that aliens outnumber soldiers, not just for the first mission but every mission, and this chart, receiving three shots in full cover from a Sectoid is already a coin flip. Their numbers (S: 75 Aim, M: +25 Crit) included for context.

The point being, knowledge of game mechanics and other tactics must be exhausted before considering the naive option of just pressing Fire. Every shot an alien takes at a soldier that is not Hunkered down in full cover is frankly almost always a misplay. The qualifier "almost always" being made only because of certain "bad" maps where high cover and avoiding LoS may not be infinitely available, such as in corridor maps: Street Hurricane, Train Station (cover is easily destroyed), Highway Bridge.

There's just more enemies

Compared to all other difficulties, Impossible difficulty simply has more enemies. Abduction missions contain 2 additional aliens for the same threat level compared to all other difficulties with Very Difficult containing 4 pods and the maximum of 3 aliens per pod, as well as removing Easy and very unlikely for Moderate to show up early. Similarly for almost all UFOs.

This may be one of the bigger contributors to the steepness of Impossible, with stat changes or reduced resources being a bit overstated. Had the first Impossible mission been 6 Sectoids, a player might get away with using 1 Grenade for 1 Sectoid of each pod, kill it, take one shot from the remaining Sectoid of each pod, and Hunker down until it moves to low cover to gamble a 72% lethal 4 man focus fire. If not, one might still win the gamble 4 against 1 behind high cover with enough retries.

It is crucial to understand that cumulative shots stack up lethality chance. More aliens means more shots at soldiers means increased lethality chance. It can not be stressed enough that the game revolves around this, reducing their lethality chance as well as possible or not giving them a shot at all, and not exactly around your own hit chances. Soldiers will always be outnumbered every mission, and so there is always the threat of taking one too many non-1%s. This will remain constant throughout Impossible with the scale only gradually tipping as the game goes on.

Patience

If anything, the most important lesson of XCOM and Impossible Ironman is patience. Here are a few commonly made choices due to impatience.

• Taking a full distance blue move without retreat options nearby, which upon pod activation will leave that soldier defenceless. This will further pressure the player to go on the offensive to try to salvage that mistake, potentially leading to a death spiral.
• Not taking a full turn or multiple turns to reposition the squad. When changing the direction of travel, it may be necessary to spend a few turns just to reposition/reorient the Squadsight Sniper(s) and/or the rest of the squad to restore combat effectiveness before proceeding.
• After combat, stepping on an unsafe tile with remaining actions. If unsure of which periphery tiles are safe, it's better to not expose any soldiers until next turn.
• Firing first without considering defensive plays, such as breaking LoS and/or Hunkering down.

Common tactical questions

Rifle or Shotgun on Assault?

Tactically speaking, Shotguns work on most Abduction maps, Rifles much better for UFO/Terror/Bomb Disposal missions. However, advanced Shotgun research is often locked behind advanced Rifle research, making advanced Rifles available earlier, as well as advanced Rifles being good enough damage wise and much safer. Despite this, consider the basic Shotgun for April even in UFO/Council missions as the Assault Rifle is too weak for the bump to 6HP Floaters and Thin Men.

For more general tactical information, see Tactical Layer and Tactics.

HQ

Engineers vs. Scientists vs. Credits

Unless you chose Europe as the starting continent for their continent bonus, Expert Knowledge, the 4 Engineers act as an investment and outvalue the §200. This is due to Workshop building/maintenance costs, excavation costs, Access Lift building/maintenance costs and Power building/maintenance costs adding up well over that amount over a few months. The only time you should really pick credits to build Workshops is when you know what you are doing with Europe, or when other factors offset the immediate cost of a Workshop, i.e. you were able to build a Thermo Generator early and therefore power/excavation is momentarily "free"/cheap, or later in the game the value of the rebate on Alloys/Elerium is considered more valuable.

It's the same idea for Scientists and their Labs, but in exchange for research instead of implied credits. If, in the mid game, you think you'll have enough Engineers for next month's Workshop, consider Scientists or credits if there is plenty of income/power/space for Labs. The research is somewhat useful in terms of keeping up for Firestorm research, if HQ is still your primary concern and not combat.

Abduction choice & Satellite placement

Every unchecked Abduction country will receive 3 additional Panic with every neighboring country on that continent receiving 1 additional Panic. Without Council or Terror missions to reduce Panic, and with every country starting off with level 2 Panic, it is inevitable to have four level 5 Panic countries (two countries from each of the two Abduction events) at the end of every month.

You can usually save all four countries during March (North America) with the Investment approach mixed with Panic approach, and with enough loot from the first Abduction/UFO. However, the Uplink after in April will provide only 3 Satellite slots, leaving one country behind. The only way to potentially save all countries is to get lucky in March or April with Panic reductions from Council or Terror missions, then lean more towards the Panic approach. Later in the game when Engineers are no longer necessary or Panic is not a concern, switch to the Research approach.

Often it's a balance between competing approaches depending on the situation.

Panic approach

• Choose the Abduction country that will yield the lowest total Panic rise and/or the least amount of level 5 (red) countries
• Deploy on red countries
• Hold off on deploying for non red countries (if an Abduction shows up on a high Panic country next month, it "saves" Panic and reduces the total rise)

Engineer/Investment approach

• Choose Engineer Abduction reward, if unavailable choose credits or ignore countries of the following
• Deploy on countries that provide Engineers and/or in a continent with implied investment benefit (Africa or North America)

Scientist/Research approach

• Choose Scientist Abduction reward, if unavailable choose credits or ignore countries of the following
• Deploy on countries that provide Scientists and/or in a continent with implied research benefit (South America or Europe)

Cancelling Workshops & Satellites

If failing a mission means lacking the required Engineers for the planned Uplink, cut your losses by cancelling the Workshop and any satellites that won't be deployed at the end of the month. You should always build satellites individually for this reason.

Similarly, if succeeding a mission with Engineer rewards means a pending Workshop is no longer required, cancel the Workshop to save maintenance costs and keep credits liquid.

Tips

• Holding off on Xeno-Biology research before building March Uplink retains 4 Sectoid corpses for their Grey Market value, which is more than what 5 Weapon Fragments would be worth, in case it is needed. It's also just better to sell corpses over the other items.
• Going for Experimental Warfare research during March allows Phoenix Cannons for April. Only one ship equipped with Phoenix Cannon is needed to beat a Large Scout. On the other hand, if there are extra funds in March, switching to Xeno-Biology research (and building Alien Containment) right after starting March Uplink will have the possibility to complete Arc Thrower research before April's UFO to start captures. Two ships with Avalanche Missiles is needed to beat a Large Scout.

Enemy Within

HQ

Due to the increased research times, it may not be such a good idea to go for Experimental Warfare research in March just for Phoenix Cannons. Without any additional Scientist rewards from Council missions or requests, Beam Weapons would come too late for May.

The reduced Uplink cost in EW, while clearly intended to offset Meld buildings, also means there is a lower barrier to building Alien Containment in March compared to EU.

See Also

• Alien Stats
• Survival Guide
• Tactical Layer
• Tactics