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Quick guide how to get the achievement without to much effort:

Required elements:
1) We need a merc with Hit and Run perk and knife - suggested one (worked for me) that is ready just after hiring is Shadow.
2) We need to make the character posses at least 18 AP on start of a turn (preferably 21+). Shadow has 13 AP just after hiring. If he is alone in the map he gets additional +4 AP. Good morale gives at least +1 AP. Using steroids +2 AP. Active resting in any city can give yet another +1. Its more than enough.
3) We need 1 strong enemy (that will not be eventually killed after 5 attacks using knife) or multiple weaker ones close to each other (closer than 5 fields). I personally did the achievement in Enemy Outpost close to Ille Morat - there are 5 hyenas closed within a fence. Once you open a fence they become hostile and its perfect place to do that achievement without trouble. It can also be done after cleaning the outpost from other enemies first.

Guide:
1) Merc with Hit and run perk start the fight being close to enemy (<=5 fields from enemy). In my case I opened the hyena cage.
2) Merc uses ALL 5 free movement possible to get to the enemy (in case enemy is closer than 5 fields from merc go there and forth to end up near the enemy after using all fee moves but without using actual AP).
3) Attack with knife (without increased accuracy) using 3AP. You should be granted 5 free movement AP again after the attack (regardless if you hit or miss)
4) Repeat steps 2-3 six times (each time using exactly 5 free movement AP and 3 AP for attacking with Knife).
5) 6x5 field movement + initial 5 free movement should give you achievement. It requires initial 18 AP for 6 knife attacks if you do everything perfectly. If you have more than 18 AP on merc you can make small mistakes in using exactly 5 free movement (you are able to use 5+1AP and still be able to attack 6 times).

Hope it helps!