Mass Effect

The weapon achievements are tied into your profile, not your game. So I was able to unlock this by starting a new game with an engineer (I wanted to get the tech achievements as well) and save the game as soon as you land on Eden Prime. There are 3 gas bags in that same area. All you have to do is kill all 3 of them, load your game, and repeat.

150 kills / 3 enemies per load = 50 reloads

Yeah its tedious, but at least you don't have to play through the game again just to try to kill 150 enemies.

If you have the Pinnacle Station DLC. There are several mission types. One of them is called Hunt and another survival. These are the two that are easiest to get kills on.

Ready up the sniper rifle and pick them off. Shoot, Kill, Repeat. It took me about 5 rounds in those mission types for this achievement.

People say that this is the most difficult gun to use, but I didn't find it difficult at all.

Just keep the rifle upgraded, and be sure to put many points into it if you are a soldier. Try to upgrade with upgrades that improve accuracy and stability.

If you are still having problems with it, save your money and buy the spectre gear sniper rifle, which has a tremendous increase in damage and accuracy, also late in the game you will find the explosive rounds upgrade which kills most regular enemies in one shot.

If you play a soldier/shock trooper, you are probably best to do this in the 40/50 level range, after you max out fitness, assault rifle, spectre skill and armour, leaving you free to get sniper rifle skill up to max by about level 55.

Get the Spectre Gear sniper rifle.
Put a Level 8 (9?) Combat Optics in it, to help accuracy.

The 2nd slot depends on how high you have Snipers. At lower levels, you probably want a stabiliser or accuracy mod.

At high levels, just stick in a scram rail.

Now the important part. High Explosive Ammo.

This gives you like 420% damage, adds force damage.

It also overheats the gun in one shot, but it doesn't matter because it also kills almost anything in one shot as well, at worst you knock them onto the floor with a sliver of health left.

It will insta-kill every Geth unit except a Geth Prime or Armature (and perhaps a Collossus). It will one-shot almost any regular organic enemy.

It also has a good radius, so anyone NEXT to the guy you are about to kill, will usually die as well, or get just as badly damage + knocked over.

I just unlocked this achievement while using my level 59 Adept on my Insanity run.

An easy place in which to work on this is during the side mission where you have to retrieve a data recorder from a downed Alliance probe. The data recorder is taken by little monkey like creatures on Eletania in the Hercules system in the Attican Beta sector. The mission is given when you enter either that system.

After retrieving the data recorder from the monkey in the mine and exiting the mine, you are presented with a small colony of approximately 10 monkey individuals. You can one hit kill the monkies with the sniper rifle and it will count towards the achievement. Much easier than trying to do it in combat, in my opinion. Plus, you don't have to worry about the other squad members kill stealing you.

Simply kill the 10 monkies, reload your save, and repeat. It'll take approximately 15 reloads, depending upon how many you can kill before the toxic atmosphere of the planet kills you.

Disclaimer: Kills carry over across all saves, and you don't have to save for them to count. This solution is for people who just want to grind the achievement as quickly as possible, even if untrained in Sniper Rifles. You must have command of the Normandy before you can follow this solution.

Killing friendly critters counts towards your kills. Go to the planet Eletania (in the Hercules system of Attican Beta) where you will find several colonies of little monkey aliens. Simply drive up to one in the Mako, save your game, and shoot each of the monkeys with your Sniper Rifle of choice. When they're all dead, reload your save and repeat.

There are about ten monkeys in each colony, so you'll need to do this fifteen times at most. Note that there are poisonous toxins in the atmosphere that will eventually kill you, but you should be able to get all the monkeys before then.

NOTE: if you are untrained in Sniper Rifles the accuracy is atrocious, if you crouch your aim is slightly improved and should be able to take out the little critters after enough tries. Also, it's worth adding "High Explosive Rounds" as a upgrade to your Sniper Rifle, this will allow multiple kills with a single shot if your lucky.

To add to |x| Suicide |x|'s solution, for those who have a high level character that is untrained in the Sniper Rifle, what I did was upgrade it with High Explosive Rounds. Switch the difficulty to casual and then go to Pinnacle Station and do a survival mode.

Just hide in a corner and enemies will run at you, duck so you are a little more accurate and try and shoot enemies as they get close. It should be one-hit kills and you can take out multiple enemies. Only took me 2 rounds of survival for about 10 minutes each to get this achievement.

Note: If you are having trouble finding a place to hide, I did this on Volcanic where there are two buildings down stairs. Go to the rightmost building and hide on the little balcony in front. It helps funnel the enemies to make it easier to get multikills.

Edit: Just a quick note, adding Explosive rounds will increase damage a lot, but will overheat your gun after every shot. It doesn't take long for the cooldown and the extra damage makes a big difference in getting more kills.

I believe I found a better location than the one https://www.trueachievements.com/ColdCyberfuNk.htm pointed out. When you first land on Virmire, you'll see 4 creatures crawling around in the water straight ahead about 220-230 meters away from the landing position (yeah, it's only one more creature, but 150/4 = 38 reloads as opposed to 50 if you're starting from scratch.) Two of the four creatures move extremely slow, while the other two move slightly faster, but it's not like they're moving so fast that you can't get a bead on them. As soon as you land, you can snipe all four of them and reload your autosave (if you have it on.) If you don't have autosave on, right after Shepard tells Joker to stay away from the AA towers and maintain evasive maneuvers, save your game. Then snipe the four enemies, load save, rinse and repeat.

This is the method I used to get the achievement and I can confirm it works.