ADVENT Mech (XCOM2)
ADVENT Mechs are fully automated units, with high health and heavy armor. Their weapons however are only slightly better than a normal Mag Rifle, but the fact that they're tough and don't need to pay attention to cover means they can easily get flanks with them. Mechs are also armed with cluster missiles that do medium damage, but have a wide area of effect and very effectively destroy cover. They also can jump, so keep that in mind if you think staying on rooftops will keep you safe from flanks.
Shredding the armor from a Mech with explosives will make it a lot easier to take down. As fully mechanized units, Mechs are immune to psionics, but are vulnerable to the Specialist's Haywire Protocol ability.
ADVENT Mechs will be supplemented by ADVENT Heavy Mechs later in the campaign. There seems little difference between the models except for increased stats, and an additional salvo of missiles.