The hostile Stalker comes in three forms - the weaker, lower-leveled Specimen d01 "Proto-Stalker", more simply known as the Proto-Stalker, the stronger Specimen d03 "Advanced Stalker", aka the Adv. Stalker, and the Archetype Stalker.
The Stalker infected units has a scythe for an arm and uses it as either a frontline weapon or a backstab behind enemy lines. Either way, the Stalker's critical rates are high, strike up to three targets at once, and have proven quite effective against armored and unarmored units alike.
Its Cleave attack is particularly deadly, especially coming from the Adv. Stalker. Though it has a bonus to land a critical against Tanks, the base damage is high enough to hurt just about any unarmored unit. The armor piercing nature of the attack pretty much ignores most armor bonuses, which tend to be useful against damage - just not in this case.
The Back Slash performs just like the Hitman's Three Handed Backstab attack, where the range is measured from the back row, starting from the Stalker's position (i.e. the Stalker cannot perform its Back Slash attack while it is positioned in the front row). Be very wary of this attack since it has a high critical chance against Soldiers and can pretty much target any unit, even those hiding behind (in front) of other units, given its tricky targeting nature.