@DavidCorben Hi! That mission was hard for me too! I barely made it with very few units.
To win the mission I did the following:
First, I advanced slowly, using flanking tactics to maximize firepower (which minimized damage taken by my units, since the faster I killed the enemy, the less their assault would last).
Second, I attacked one unit at a time, preventing the enemy to get too close and in numbers larger than what I could handle.
Third, I destroyed turrets using reapers in first person mode, positioning
myself out of their range.
Fourth, I explored the area using a fast unit, while keeping my squad behind in the formation aforementioned. Then I returned that unit to my squad, maintaining my unit density as high as possible.
Fifth, I never retreated as a squad; I always retreated while scouting. Instead, I moved my squad inching my way through the areas previously explored.