The First Tactical Engineer Ivan encounter, is on Mission 3. A Tactical Engineer along with some Golem Soldiers are sent into to stop Ivan and Delta Force from opening a door, Ivan and Delta Force are tasked with holding them off well Bob opens the door.
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Tactical Engineer Gunners: Gunners are equipped with Mini Guns.
Tactical Engineer Knights: Knights are equipped with Large axes.
Tactical Engineer Sentinels: Sentinels are equipped with Chainsaws.
Tactical Engineers are equipped with heavy power armor making them harder to kill than other units of the S.O.D army, they are also really slow due to the armor.
Two Melee Shots will kill most Tactical Engineers. Melee Shots seem to greatly injure Tactical Engineers much more than the attack should normally damage.
In Devil's Third Online, it seems that one of the basic X-Gear "weapons" available is the ability to change into a Tactical Engineer with a chainsaw for a limited amount of time, able to swing the chainsaw plus with a charging spin move.