Unless employing a very specific strategy, it's nearly impossible to get zero civilian casualties in UNC: Beseiged Base. Although Mass Effect Legendary Edition prides itself on being a game that is all about player choice, talent comes in at various moments as well. Yet, it would take an absurd amount of skill and luck to save every addled civilian on Chohe.
Most players either drain several hours trying to get a perfect firefight with no luck before accepting their fate and moving on. That's not the worst thing that can happen, Admiral Hackett seems to understand that the situation was impossible all along. But Shepard makes a living out of doing the impossible and beating every set expectation, this mission doesn't need to be any different.
Preparing For The Fight
Half of the battle is won before the doors to the facility are ever opened. Because enemies will be mingling with innocent civilians, it's best to devise a strategy that keeps collateral damage to a minimum.
Bring squadmates that don't use area damage skills. Ashley is the best choice, Garrus and Wrex are decent as well, especially if they've been leveled up to do physical damage. Liara should probably sit this one out with the large area of damage done by her biotic abilities.
For those who don't like the idea of leaving their favorite companions on the Normandy, disable the ability for squadmates to use abilities on their own in the options menu. That way, the player can control their moves and can limit their uses to the ones that don't do damage to clusters of enemies.
Ditch the shotguns and assault rifles in favor of sniper rifles and pistols. These weapons are more precise and have no splash damage. There is still an off chance that a teammate will hit a bystander, but this at least minimizes the odds of this happening.
Safely Engaging The Combatants
Save before the fight. This strategy is wildly successful, even on Insanity difficulty, but that doesn't mean it's impossible to fail. This is a single battle, so reloading to just before it takes place will save an entire save file.
While it's natural to leave the squadmates set to follow Shepard, they're too much of a liability. That's not to say they can't help, but any companions should be placed in the initial room, behind cover. They can see into the area where enemies come from, but any further than that and they could kill somebody they shouldn't.
The single most important part of this entire guide is to lure the biotics from the second room into the first room. Open the door to the area, fire a shot to get their attention, and then run back.
There are no civilians in this first room and the team can engage in no-holds-barred combat here. Additionally, the enemies will funnel down the hallway and it's easier to line up headshots and dodge biotic attacks.
Congratulations on turning this tough mission into a cakewalk! Head back to the Star Map and Admiral Hackett will express his gratitude and dismay.