A large quadraped mech, with a Sectoid pilot: a heavy unit with thick armor plating, heavy weapons fire is required to destroy it in a head-on fight. Or, use targeted shots to destroy the canopy, then the pilot, or somehow get a view of the back of the fuselage (flank it or distract it), and target the plasma pipes on the rear.
It is essentially a mobile Gun Turret, with a melee attack.
Salvage Report: Sectopod
While not a living alien species, the Sectopod poses a new danger. Standing 20 feet tall, this heavily armored fighting robot is equipped with heavy weapons atop multiple, all-terrain legs that can be used as impromptu melee weapons.
Sectopods are controlled by a living Sectoid encased and sustained within the armor. These “pilots” are unable to leave the Sectopod- in many cases their containment tanks are welded shut- and may actually be a carefully bred subspecies developed solely for this purpose. The pilots’ loyalty would have to be unquestionable, and while they do not wear the slave collars of their brethren, they bear significantly more intrusive neural implants.