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. When you go further to Mutons, Beam Weapons again require at least two shots ignoring crits. 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.

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.

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)
# of 65 Aim shots	C	I	C	I	C	I	C	I
1	56.33%	47.67%	31.5%	18%	17.5%	10%	30.33%	15.17%
2	81.68%	75.62%	54.9%	40.05%	32.5%	21.25%	53.77%	37.23%
3	92.53%	89.34%	71.11%	58.86%	45.16%	32.5%	70.30%	56.78%
4	97.01%	95.53%	81.87%	72.88%	55.70%	43.05%	81.36%	71.54%

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 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 active 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 four shots in full cover is already nearing a coin flip; Not to mention that for Sectoids, these numbers are inaccurate. They have 75 Aim and Mind Merge crit chance boost. Three shots may be significant enough.

The point being, knowledge of the 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 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 hit 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 tactics, see Tactical Layer and Tactics.

HQ

Engineers vs. Scientists vs. Credits

Unless you have the Europe continent bonus, Expert Knowledge, to significantly reduce the cost of Workshops, 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. The only time you should really pick credits is when you know what you are doing with Europe, or really late in the game when Engineers don't matter anymore and the game also doesn't give you Scientists.

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, then consider Scientists. The research is somewhat useful in terms of keeping up for Firestorm research, if HQ is still your primary concern and not Combat.

Panic

Panic management can be one of the biggest pains during the starting months on Impossible. The 2 Abduction missions that take place will leave the 4 unattended countries with a level 5 panic meaning that those 4 countries will leave the project at the end of the month. There are two main methods of dealing with this issue:

• Let It Ride - build only 1 satellite during March and launch it over the country that offers the biggest funding or to be able to secure a continent during April. This is the usual method if you are planning to do an April Base Rush strategy, where the main goal is to assault the Alien Base as quickly as possible for the panic reduction and the loot.
  • Advantages - You'll be able to use the credits to get the OTS/Foundry/Alien Containment up and running and advance through your research or get some squad upgrades or S.H.I.V.s quickly.
  • Disadvantages - Your funding will not increase much in April, forcing you to sell a lot of alien artifacts later on. You risk losing one or more useful continent bonuses.
• Satellite Of Love - build 3 sats before March 10th and get 5 engineers as quickly as possible before the 16th to build the 2nd Satellite Uplink. This allows you to cover 3 countries with sats and reduce their panic and, if you are lucky, a Council mission will take place in one of those level 5 countries, meaning 0 defections on March. This method is usually the first step into the Satellite Rush strategy, where the main goal is to get at least 8 satellites on orbit as quickly as possible.
  • Advantages - a lot of needed credits in April and keeping most or all options available regarding continent bonuses for later on.
  • Disavantages - you'll need §200 by the 10th to pay for the sats and an additional 6 engineers by the 16th. While it is easy to get the credits, it's also possible not to get an engineer reward on the 1st Abduction mission, sending this plan crashing back to Earth. And even with the first engineer award, the 2nd Abduction mission may not happen before the 16th, but to anticipate that possibility you can always try building a Workshop as quickly as possible. All of this means that there will be few credits left for any squad upgrades.

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.

See Also

• Alien Stats
• Survival Guide
• Tactical Layer
• Tactics