What am I doing wrong in this ME1 mission?

I think activating your barrier can pose a problem... so don't. What I do when I'm low on the gas grenades is just run in quick, punch a colonist unconscious and then run back into cover to regen shields and health and dissipate the fatigue, if necessary. Shepard will punch automatically if you sprint right into a colonist.

I also tend to order my squad mates into cover well back of the colonists, so the squad doesn't start to take damage and get a notion to defend themselves (because I've found that occasionally they will kill colonists by using their defensive abilities even when ordered not to fire on them).

I've also found that sometimes punching the colonists in the "wrong" order can result in one dying inexplicably (this seems to be a bug)... but then I just reload and try knocking them out in a different order. This seems to happen if I punch out Ledra before punching out the colonist near him (she dies) and sometimes when I'm just punching out the next group of 4 one at a time (although I can't remember the precise order that triggers one dying.

Also, Ledra, Hannah Murai, and Fai Dan's second in command (can't remember her name) can be really tough to punch down; and sometimes they can get stuck in the walls and you may wind up killing them then. If one does get stuck, and you have a grenade left, you can throw a grenade at the wall they are stuck in and they will go down just fine then (so I generally start using the punching while I still have one grenade left - just in case. If you're right out of grenades, it might be a restart since you can't finish the mission if one is awake but stuck in a wall.

Change the dificult to veteran. On normal and below, they die like flies.

Since you are a Vanguard, you have Throw and Lift. Note that these powers can kill colonists, just like thorian creepers.

I just realized one other thing I did forget to mention... make sure you have your squad mates abilities are not set on "active" or your squad will usually kill a colonist or two even if you're good about ordering them to stay back behind cover.

Well I fnished the mission with saving all colonists ... but it still irks me that I hat this much problems getting through like that? While my previous PT's required maybe 2 or 3 reloads? Oh well.