The Mechtoid is a Sectoid in a Mech suit. It is a robotic unit armed with twin Plasma Mini-Cannons and can fire twice in a turn if it doesn't move. Mechtoids don't utilize cover but have a solid block of health (20 HP on normal). Sectoids can Mind Merge with a Mechtoid and give it a Psi Shield with 6 HP durability and reduces damage against the Mechtoid by half as long as the shield is up; sufficient damage can break Psi Shield. Killing the Sectoid merger cancels the shield and causes 3 damage to the linked Mechtoid.
Mechtoids first appear during the fourth month (June), or August if the "Marathon" Second Wave option is enabled. There is a limit of one per mission for first month (does not apply to Impossible difficulty), and they are most prevalent from July to September (Marathon: October 2015 to March 2016). In October (Marathon: April 2016) their chance of being generated declines. Mechtoids can be deployed for several mission types, but are not encountered during terror missions.
Mechtoids utilize new Plasma Mini-Cannons in combat. These weapons have base 7 Damage on Normal difficulty but -1 Damage on Easy, +2 Damage on Classic, and +4 Damage on Impossible. Much like the Cyberdisc and Sectopod, these integrated weapons cannot be captured and are destroyed when the Mechtoid is killed.
A Mechtoid will typically split its fire between two different enemies when utilizing Plasma Barrage unless there is only one valid target.
These enemies are often quite tough and can pose a major threat if left alone, they almost always have one or two Sectoids with them; during later portions of the game, they are occasionally paired with a Sectoid Commander.
A common and effective strategy for dealing with a Mechtoid is to use the first round of combat to eliminate the accompanying Sectoids, this will prevent the Mechtoid from (re)acquiring a shield which will make it much easier to deal with. Afterward the simplest way to take one down is to have the squad attack it until it is reduced to 12/18 or less health and then have a MEC with a Kinetic Strike Module finish it off with a good punch; if this is not an option concentrated fire from an entire squad of beam weapon wielding soldiers is usually enough to put a Mechtoid down in one turn.
An similar strategy is to allow the Mechtoid to acquire a shield, then kill the Sectoid giving it. On the Easy and Normal difficulties (and Classic if the MEC Close Combat Foundry project is completed), this will leave enough health for the Mechtoid to be finished off with a single knockout punch from a MEC Trooper. This strategy is more efficient, as it consumes fewer actions from your soldiers.
However, on Easy difficulty the Mechtoid only has 16 health, so the MEC Close Combat foundry project will allow a MEC trooper to kill an uninjured Mechtoid in with a single blow.
A Mechtoid generally tends to move away after it's punched (if it's not eliminated, obviously), sparing the attacking MEC a close-range plasma blast - or, likely, two.
Unlike the other (fully) robotic aliens, Mechtoids are susceptible to psionic attacks including Mind Control.
Note that an attack (such as a rocket) which simultaneously strikes a Mechtoid and kills the Sectoid that was Psi-shielding it will not have its damage reduced by the Psi-Shield.
If you are collecting Meld, remember not to kill a Mechtoid with explosions.
The developers had originally planned to use the Muton as the basis for a Mech suited alien but the idea was dropped due to the Muton's already imposing size; the Sectoid was chosen as the replacement because it was imagined as the unit who would most desire a Mech suit.
Dealing a killing blow with the Kinetic Strike Module will play a unique cutscene where the Mechtoid attempts to resist, but is held off and then finished.