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.

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 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 you, 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.

Tactics

See page: Tactics

Don't 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.

Make Your Own Luck

If failing a mission means lacking the required engineers for the uplink/satellites - cut your losses by cancelling workshop/satellites (you should always build them in batches of 1 for this purpose) that won't have an effect by the end of the month.

Patience

If anything, the most important lesson of XCOM and Impossible Ironman is a lesson in 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 full offense to try to recover from 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.

Squad Equiping

1. When equipping during the initial months, the choice is between grenades or Nano-Fiber Vests. The first means the ability to remove cover, and weaken/kill single or clustered aliens. The second the ability to survive a single Plasma Pistol or LPR shot.

Aliens

Sectoids

• 7 tile blue move.

Floaters

• 7 tile blue move.

Thin Men

• 9 tile blue move.

Outsiders

• 11 tile blue move.
• If not surrounded from two orthogonal directions at distance on activation, the Outsider will position offensively, allowing itself to be easily flanked unlike other non-flying ranged aliens. Or, at the very least, be in close proximity for an explosive.
• Also continues positioning aggressively after activation, though continued combat with an Outsider isn't preferable considering their shots against high cover have a 50% hit chance.

Chryssalids and Zombies

• Chryssalids have a 13 tile blue move. Stay 14-16 tiles away (firing range) or more if you have the option.
• On their third turn, Zombies will erupt and be replaced by a white Chryssalid utilizing that turn. Without proof of when a Zombie spawned, don't risk it. If you can, stay at least 14 tiles away anyway.

Sectoid Commanders

• Similar to Outsider, but it seems once they, Outsiders or Sectoid Commanders, start spawning in pairs, only one goes offensive, the other positions normally but within vision.

Muton Elites

• Can't be killed with a single plasma shot that isn't ability enhanced (i.e. Headshot, Shredder, Bring 'Em On) since it will deal a maximum of 15 damage with a critical hit.

Heavy Floaters

• Can't be killed with a single plasma shot that isn't ability enhanced (i.e. Headshot, Shredder, Bring 'Em On) since it will deal a maximum of 15 damage with a critical hit.

Enemy Within DLC

• Muton Elites Health Points have been increased to 18 on Classic (from previous 14) and 20 on Impossible difficulties (up from 18).
• No longer possible to abuse the Overwatch exploit to get aliens to freeze.

See Also

• Alien Stats
• Survival Guide
• Tactics