Scouting

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When shot at, aliens have a nasty habit of shooting back. Their reaction fire is fast and deadly, but there are several ways to avoid reaction fire completely. The two most important things to know are:

• Mutual surprise. If X-COM and alien units spot each other simultaneously, the aliens will not fire first.
• Line of sight. Aliens cannot return fire upon units they cannot see.

Therefore, the soldier who sees the alien, should not be the soldier who shoots the alien. You should switch to units which have line of fire to the alien but not line of sight, and have them take it out without fear of reprisal. (Generally, if a unit can see an alien, the alien can see the unit -- although there are sometimes blind spots -- and at night, aliens can see 20 squares away while X-COM units can only see 9.)

Thus we have the Scout/Sniper teamwork tactic.

Scout/Sniper Tactics

Consider having your soldiers who have good Reactions (above 50) and Time Units but are poor in Firing Accuracy (below 50) and other stats be your scouts. While soldiers with high Firing Accuracy should be snipers.

When using a scout, creep them forward slowly, always finishing their turn crouched and behind cover to avoid being noticed during the alien turn. Before the scout turns corners, the Sniper should catch up and get a good angle on the corner, allowing them to still be able to cover the scout. If you have multiple scouts and/or alien contacts in the same turn, stop advancing your scouts when your snipers run out of TUs.

Preferably you should have multiple snipers who stay within (relatively) safe cleared areas, moving only if the Scout has completed their movement with no alien contact. If a sniper is out of range (20 tiles) they're safe from return fire. Units which don't have line of fire may still be able to toss grenades.

If scout spots an alien who is facing them, Do not move them unless they can move one tile to break line of sight. If the scout spots an alien at the same time that the alien first spots them, the scout will not be fired on until their next action due to the Mutual surprise mechanic. If the scout can move out of the alien's visual range, then they can become an additional sniper and safely fire upon the alien.

This tactic is best suited to the middle third of a battle, after LZ deployment and before storming the UFO, when there's a lot of relatively open ground to clear. See also Sweeping the Battlescape.

Smoke cover permits the use of Scout/Sniper tactics over short distances, or in confined areas. If a scout wandering through smoke spots an alien, have them freeze (or move one square back out of the alien's visual range), then have nearby soldiers fire at it through the smoke cloud. This works well during deployment from the transport -- if you have dropped a smoke grenade at the base of the ramp, snipers can hide in the cloud while scouts beat out the surrounding terrain.

More Tips

• Don't fall into the trap where, when your scout gets shot, you walk another soldier to the same spot to see if you can spot the shooter. The enemy has the TU advantage, the established line of sight, and probably good cover to boot.

I've lost three soldiers rounding the same wheel of the Skyranger before it occurred to me this was unproductive behavior. --JellyfishGreen

• If there's a lot of open ground with nothing for your scouts to hide behind, consider having them e.g. move out ~6 tiles, then back 3-4. This ensures that an alien will have less TUs to shoot if it subsequently does move so as to see your scout. If you do this with scout pairs, they can switch off with one doing the running while the other rests (just moving up 3-4 to be next to the runner). The resting scout is also ready to take a couple of reaction shots, if needed.
• When all that separates your soldier and the alien is a closed interior door, as in our favorite Large Scout, try this trick with Heavy Plasma: Move a second soldier beside the door, where he's not in the line of fire. Have him shoot the door away. (It takes an average of 9 hits; see Destroying Terrain.) This leaves your first soldier facing the alien, and you can take your auto-shot before the alien can get in reaction fire. You can even place a third shooter beside your main shooter (on the far side of the door), where he might be able to e.g. look down a hallway angling away from the door. from StrategyCore
• Snipers can get best results with a series of Snap or even Aimed shots, if they are out of visibility range. This also reduces friendly fire risk. At closer range, though, Auto fire is better to keep the alien from getting off a reaction shot. Statistical calc of Snap vs Auto by Hobbes on StrategyCore
• Whenever possible, try to walk around corners at a 45-degree angle. If you can spot an alien at the same instant that it first spots you, it will not fire upon you. So instead of walking blindly around a corner and then turning, approach it at a diagonal. This will generally allow your troops to see what's around the corner as they round it. For more information see line of sight.
• As the game progresses, eventually Aimed shots become almost guaranteed hits for long distances.

• Using a Heavy Plasma (110% Base Aimed Shot accuracy), a Sniper who is kneeling (+16% accuracy on aimed shot), with an accuracy of 80 will have a 101% chance of hitting on target with an aimed shot. Since accuracy increases from shooting experience, a force can easily have 1 or 2 snipers with perfect or near-perfect accuracy. However, due to the variability of weapon damage, a single Heavy Plasma hit will still fail to kill a lowly Sectoid about 15% of the time, and a Muton about 60% of the time.

• The standard Rifle and Small Launcher also sport 110% aimed shots.

• The Rocket Launcher also benefits from aimed shots. It begins with a base aimed accuracy of 115%, which turns into 132% accuracy when kneeling. With a weapon accuracy this high, your soldier only needs to have an accuracy of 76 or greater to achieve almost perfect hits.
• Another favourite long distance weapon is the Plasma Rifle. Though its aimed shot is not as accurate as the Heavy Plasma, its snap shots have a base accuracy of 86%. Combined with kneeling, this becomes an impressive 98% accuracy. The lower cost of snap shots allows a sniper to fire off 3 shots per round, which makes up for the lower damage and increases the amount of hit experience that you can gather per mission. However, as the game progresses, aliens stop carrying plasma rifles in favor of heavy plasmas, and you must start manufacturing its ammo yourself.