This guide wont make the achievement any easier, you need to be lucky and patient.
So, in this guide am just telling you my experience of how i got it :)
Things might me different on your play through but i'll tell you the main things to do and you'll need to improvise.
Good luck,,, you'll need it

The achievement is in mission III of Saladin's Conquest DLC.
Basically you'll need to capture all the wells in the map , but its more complicated than that because once you capture one of the wells the needed water count bar will fill faster and the quest will end and begin the next one, after that if you captured the rest of the wells you wont get the achievement, every well is heavily guarded by 4 units or more and the enemy will keep sending reinforcements.
So, you'll need to do some pro gamer moves.
Remember to keep saving the game after every move you do.

Once you start the quest they'll ask you to capture a well (Don't worry the bar wont fill quickly from this one).
On your way you'll see a caravan protect it.
After you capture the well there are 2 villages to be captured.
After you capture one of the villages a side quest will pop up about protecting another caravan
Do it, its so easy to protect, just place a trap on the road and move your grenadiers up to the cliff and let them do their job.
you can move Gokbori and the horsemen to the road if the grenadiers missed someone.
SAVE THE GAME NOW

After that go capture the next village and protect them with your life.
Build the three watchtowers on both of the villages.
SAVE THE GAME NOW

First, build a barracks and houses an train any unit you like (i recommend a militia and grenadiers) just to protect your villages.
after you set your units in a good positions SAVE THE GAME and take your time to build all Archery towers, stables and black smith (in order to build those you need to upgrade the technology tier)
It will take a long time to gather resources to build and upgrade and by that i mean IT WILL TAKE FOREVER but yeah small price to pay for salvation (it took me about an hour).
After you have built all archery towers, Stables..... now start to make your army
Leave a one grenadiers unit in each village (A,B) to protect them (lvl them up)
Release all your other units one by one and replace them with grenadiers and train one lancer unit and lvl them up
Now you should have (Gokbori (the hero),8 grenadiers, one lancer unit)
Now you are ready to kick some fluffy butts
SAVE THE GAME NOW

It's the nearest village to the base (Village D)
Their is a 4 units covering it (2 archery units, 2 infantry, 1 infantry reinforcement unit will join the fight)
you can split your army to attack from both sides.
Use the grenadiers to kill the archers and use the lancers and Gokbori to protect them when the infantry attacks.
after defeating all units destroy the village but DON'T CAPTURE IT just leave a one grenadiers unit near by to prevent the enemy from recapturing the village.
Now you can send your units one by one to the base to heal and replace casualties.
SAVE THE GAME NOW

It's the one on the bottom of the map ,village (C)
Like the last one, get rid of all enemy units and destroy the village you can capture this one because the well isn't in it, you can find it on top of the village with 3 units they are easy to kill by grenadiers, now they will ask you to repair the well DON'T you'll repair it later in the right time.
If any thing went wrong you can reload last save and try again
Also you need to leave a grenadiers unit near the village to prevent the enemy from retaking it now you can heal and replace casualties in this village.

It's the one on the left side of village (C), village (E)
Nothing special about it, just kill all units and destroy the village but DON'T CAPTURE IT
SAVE THE GAME NOW
I don't think you need to leave any unit near by, in any case if any thing went wrong reload the save and make things right

This one is a bit tougher than the last ones so you need to reassemble all you units (except the ones protecting Village D,C)
I played this mission twice before writing this guide and every time i played it it was a bit different, but the main thing to do is killing all units and destroy the village
SAVE THE GAME NOW

Here we come the hardest part, you'll need to be fast AF.
Save the game every time you think you did something right....
Also this one is a little bit randomized...
Gather all you units (No exceptions) and attack the village from top and bottom side and destroy it and Capture it.
The well is located down in the pit it's guarded so you'll need to clear it out.
If any thing went wrong you can reload the last save......
Now the enemy will keep sending units so now you'll need to be really FAST

Now all villages are destroyed just need to be captured.
Send one unit to every village you destroyed capture them at the same time (their is wells need to be prepared)
Before you capture them save the game if any thing happen and the achievement did't popup.
It took me about 3 hours to get the achievement not considering the 3 hours just trying to get it.

And FINALLY
Congratulation, you got the rarest achievement in the game :)
(According to AStats only 21 one got this achievement in the time i wrote this guide)

As i said things might be different in you play through, that means maybe something will happen in your play through didn't happen to me....
so you need to keep paying attention to every things happening...
I hope this guide been useful to you, and thanks for reading it.
If you notice any mistake please report it to me :)

If you like it leave a thumbs up :)

Techno..