Mass Effect 3

On the second mission, the one that takes place on Mars, you'll eventually run into guys holding big shields, called Guardians. If you look closely, you'll see a small slot in their shields that they use to look through. You must kill 10 Guardians by shooting through this slot. It is quite easy to farm this achievement. When you first run into them, there will be two of them. If you are a class that can use a sniper rifle, I advise using this as it usually kills them in one shot. Otherwise, use the most accurate weapon your class can wield to shoot through this slot. Once you've killed both of them and made sure the little progress bar shows up saying you did indeed kill them through the slot, either move forward a little and die and choose to resume, or reload the autosave manually. Either way, the autosave takes place right before where you killed the two Guardians. Keep repeating this until you've killed them 10 times and you will get the achievement.

Guardians are enemies that Cerberus will throw at you in both single- and multiplayer. They are identified by the large shields they will hold in front of them to guard themselves from damage... in theory.

You will need to land ten fatal headshots on Guardians through the 'mail slot' (the little slit toward the top of their shields that they use to see through), while the Guardians have their shields lowered and are (most likely) advancing toward you. It is possible to knock them off-balance, so if you see them stagger a bit and wave their shield wildly your headshot will not count. These can be accomplished in singleplayer, multiplayer, or any combination of the two.

Only Sniper Rifles are really effective at this. Even powerful single-shot weapons the Mattock aren't going to cut it unless they're heavily upgraded and modified, but at that point in the game you'll probably have the achievement already anyway. I recommend using the Valiant since its three-shot clips allow you to get more than one shot in if you're using an ability like Adrenaline Burst. Additionally, for anyone who can lower the difficulty (aka, not an Insanity run) to Casual will find a ridiculous aim assist will allow you to perform headshots with methodical frequency. Feel free to make use of it.

It is also of note that a frontal headshot through the slit isn't always necessary. Not that it's a difficult achievement to get anyway, but at least two of my kills were headshots achieved through the side, when the Guardians were pursuing my squadmates. Xaero Hour has also confirmed that this is possible to gain progress for with a side-shot headshot. This may help you if you're playing on a higher difficulty and don't want to face them head-on yourself, although the exact angle or specifications that make it no longer count toward this are still unknown.

Guardians are enemies that Cerberus will throw at you in both single- and multiplayer. They are identified by the large shields they will hold in front of them to guard themselves from damage... in theory.

You will need to land ten fatal headshots on Guardians through the 'mail slot' (the little slit toward the top of their shields that they use to see through), while the Guardians have their shields lowered and are (most likely) advancing toward you. It is possible to knock them off-balance, so if you see them stagger a bit and wave their shield wildly your headshot will not count. These can be accomplished in singleplayer, multiplayer, or any combination of the two.

Only Sniper Rifles are really effective at this. Even powerful single-shot weapons the Mattock aren't going to cut it unless they're heavily upgraded and modified, but at that point in the game you'll probably have the achievement already anyway. I recommend using the Valiant since its three-shot clips allow you to get more than one shot in if you're using an ability like Adrenaline Burst. Additionally, for anyone who can lower the difficulty (aka, not an Insanity run) to Casual will find a ridiculous aim assist will allow you to perform headshots with methodical frequency. Feel free to make use of it.

It is also of note that a frontal headshot through the slit isn't always necessary. Not that it's a difficult achievement to get anyway, but at least two of my kills were headshots achieved through the side, when the Guardians were pursuing my squadmates. Xaero Hour has also confirmed that this is possible to gain progress for with a side-shot headshot. This may help you if you're playing on a higher difficulty and don't want to face them head-on yourself, although the exact angle or specifications that make it no longer count toward this are still unknown.

This achievement can be obtained by playing multiplayer.

Either start up a match or do a quick match with Cerberus as the enemies and it will spawn guardians after a few rounds. Choose an infiltrator class and equip a sniper rifle.

When a guardian is walking towards you, cloak and aim around the corner. In most cases the guardian will still be walking towards you and you can take your time lining up your shot.

This achievement can be obtained by playing multiplayer.

Either start up a match or do a quick match with Cerberus as the enemies and it will spawn guardians after a few rounds. Choose an infiltrator class and equip a sniper rifle.

When a guardian is walking towards you, cloak and aim around the corner. In most cases the guardian will still be walking towards you and you can take your time lining up your shot.

In the second mission, MARS: PROTHEAN ARCHIVES you will get into a battle with 2 Guardians down a long hall. This is immediately after the small room with the Weapons Bench. Just sniper these 2 through the small slot in the shield and then reload and do it again. Repeat until 10 are killed.

"Mail-slotting" is killing the Guardians (the chaps with the shields) by
shooting them through the viewing slot in the shield....hence; Mail Slot.

A straight forward way of doing this would be :
If you haven't already - Download the From Ashes DLC

Go to the Priority:Eden Prime Mission

***no spoilers here***

just after you find the 2nd video/cut-scene info
(unmissable)

A
STAY in that room
B
Position your squad members so that they don't interfere,
(remaining in the room with you is fine).
C
two Guardians will very slowly creep towards you as part of the first wave...
D
mail slot them and then allow your self to be killed
E
save puts you right at A above
F
repeat until the Achievement pops

The particle rifle you get in the from ashes dlc works well for this. Focus on the slot and hold it till he dies. It counts most of the time.