The most horrible anti X-COM weapon of the game. It is basically an organic launcher for a sentient form of living missile which erupts in acid and enzymes upon exploding on target. The result is often a dissolved heap of flesh of an agent with no equipment, with the only form of defense being the personal shield of aliens. The accuracy of the weapon is its frustrating effect, as it ALWAYS hits an agent on open ground, making the combat a matter of hiding and seeking unless the agent is cannon fodder.
While X-COM engineers cannot reproduce this weapon, it is a strangely effective weapon when used against aliens and enemy guards. Keep in mind that the whole purpose of this weapon is to cause damage through object destruction. This means it's useless when you want to gather equipment. But if destruction is your whole objective, go for it!
Official Entry: "This weapon is essentially an organism that is genetically engineered to launch a living missile which flies directly towards its chosen target. The missile then erupts and a foul smelling sticky goo smothers the unfortunate victim. The substance is a combination of enzymes and acids that can erode metallic or organic compounds. Unfortunately this weapon is not very effective against Aliens and we cannot replicate it."
An entropy launcher is simply a launcher. All launchers have a homing ability, so your agent's firing accuracy is irrelevant. Once fired it is not, like any launcher, limited in turning; it will hit you no matter the angle as long as a free path exists for it to turn. In real time combat it is possible, although particularily hard, to dodge it like all other missiles by standing upright and dropping prone just before the missile hits, in which case it will attempt to circle a 270 degree turn and come back from the side. Unlike direction changes right after firing, this turn requires much space and it will usually impact a wall before coming back.
The missile will only directly damage your agents' health through impact damage. Impact damage against Disruptor shields saps 1/3rd of the shield battery per missile, and if it destroys the shield, the target will be covered in enzyme! So even if you do not use the missiles for the enzyme effect, the launchers are useful for rapid destruction of enemy shields.
The major damaging effects of the entropy missile is done through the enzyme. This works by destroying the durability of all of your carried items. Once the enzyme has eaten through the objects, they are destroyed. When a grenade or weapon is destroyed, it will explode, and this is the real threat from the weapon. The only known objects to have any chance of surviving one coating of enzyme would be X-COM Disruptor armour parts. Note however that the armour will be severely damaged, and further damage could destroy some of the segments.
Because the enzyme only targets objects, the best way to survive an entropy missile attack is to drop everything and strip off your armour. When your agent stops smoking after a few seconds, the enzyme will have worn off. This method of saving your soldier is by far the most effective, but it also puts your agent in an incredibly vulnerable position.
If an agent is covered in enzyme with their equipment, if they are near an exit, use it. This will save the agent and their equipment. Otherwise, you need to drop everything they're carrying that can explode and run away from it, fast. Unless you have some replacement armor available, you will probably need to evacuate the agent anyway, since an unarmored agent is very vulnerable.
There are a few weapon loadouts that will not be lethal to the agent if hit by enzyme, for example smoke and gas grenades and stun grapples. If used against the aliens, they are only useful against Anthropods and Skeletoids.