I've advanced somewhat into the game (I think I'm past the first half, have done the Alien Base and 7-8 missions after it), and I'm still puzzeled about which aliens I can capture and under what conditions.
Here's what I've learned:
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Some aliens are completly un-capturable/un-zappable, like the alien disk
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Some need to be real low on life to be zapped (like Mutons must have less than 3 life)
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for some aliens it seems (and I might be wrong) that given the same species and the same amount of life, sometimes the zap is succesfull, sometimes it's not (like the human looking alien, with 3 life, sometimes I zap it and it goes down, sometimes it doesn't work)
My question is then:
What are the unzappable alien species, and for those that are zappable, which ones need special conditions for zapping and what are those conditions?
Best Answer
None need special conditions. The less health they have the more likely you are to succeed. No conditions result in a 100% capture chance as far as I can tell (the highest I've seen is 95%, with the enemy at 1 health and the advanced arc thrower researched at the Foundry). The unzappable ones are Chryssalids, Drones, Cyberdisks and Sectopods, as well as the Mechtoid and Seeker from Enemy Within. Another upgrade at the Foundry will give you the ability to use the Arc Thrower to "reprogram" drones to fight for you, but they will explode at the end of the mission, you will not be able to interrogate a drone no matter what you do.
I find attacks that deal a set amount of damage (grenades, rockets, first psionic ability) and low-yield attacks (pistols) to be great for capturing, as you can bring a target's health down in a controlled manner without risking its death to maximize chances of a successful capture.