Good areas to find enemies are around the Soviet Installation enemies can be found at the Sawmill, north east of the sawmill near a cave entrance, and East of the Sawmill when heading towards to old mine as well as enemies spawning on the upper levels of the bulding.
Another location for enemy spawns is just before you reach the research base usually 4 or 5 enemies spawn here and finally The Lost City the camp-fire just after the first gate you destroy with a trebuchet 4 or 5 deathless spawn here.