THE WITCHER 3: WILD HUNT
78 Achievements
The Enemy of My Enemy
Use the Axii Sign to force one opponent to kill another. Do this 20 times.
How to unlock the The Enemy of My Enemy achievement in THE WITCHER 3: WILD HUNT - Definitive Guide
It can be levelled up 3 times which will make the turned enemy do 60% more damage than it would have done when it was trying to kill you.
I find its best to do this when an enemy is in amongst a few of his friends, because he just goes straight into attacking them.
A good place to do this is at the monsters nest that is north of Mulbrydale (has loads of level 6 Nekkers). If you use Axii on one of the Warrior Nekkers then it easily dispatches the run of the mill kind and,if you have the puppet Axii leveled up to level 3, by the time the possession runs out your able to possess either it or another nekker again to do it all over again
Good luck :-)
Edit 1: As pointed out by Sneakystabbalot, make sure that once you've unlocked the puppet skill (and equipped it) that you hold RT to activate it rather than the usual tap of RT to stun the enemy.
I'm not sure if it takes the same amount of time at each level of puppet to control the enemy but it takes about 3 seconds of continuous holding for you to take over the other person at level 3, so make sure your far enough away from archers and other enemies before you try it.
In the Character Upgrade screen;
*select the Signs page (blue diamond marker)
*In this first row you have to spend 6 Ability points - can be in ANY Sign you like
*Once your 6th point is spent the 2nd row unlocks
*Goto the far right column (Axii sign) and put your 7th Ability in the 'Puppet' skill
Now, during your travels if you HOLD you cast the alternate Axii Sign, in this case you aim at the foe you wish to possess, let go and for a brief period their health bar is white - it is now that they kill on your behalf and contribute to your total kills required.
This works best when the enemy your targeting is close to others. If you use Puppet on one whose off by himself he'll over just stand there and not attack anyone else.
Beyond getting the achievement itself (which isn't too hard), this ability is extremely useful for getting through a lot of battles, particularly earlier in the game (when you're weaker) on harder difficulty levels.
I'd say what helped me the most was to utilize the hills and counters. If you counter them on a hill, you will kick them and it gives you some breathing room to actually cast on the guy you need to. (figure you at least hurt the one you're puppet is killing with a hit or two)
Before you can attempt this, you must first unlock the "Puppet" skill from the Axii skill tree (and preferably max it out). When using Puppet, make sure there's at least two or more enemies present in combat. As such, groups of humans/bandits, nekkers, drowners and ghouls work well when going for this achievement.
First, weaken one enemy so that they have very little health left. Now cast Axii's alternate Sign mode, Puppet, on another enemy in the group and he/it will quickly dispose of the weakened enemy. It might be easier to work on this if you're only facing two opponents at a time rather than a larger group. Either way, once an Axii-possessed enemy has killed another enemy 20 times, this achievement will unlock.
You will need to unlock the "Puppet" skill from the Sign tree and equip it in your skills. This is one of the best Signs to use in Death March difficulty, as it makes your opponents fight each other.
Once you have it equipped, find a group of enemies and weaken one enemy, then cast the alternative Axii sign (hold for a split second and release instead of tapping) on the other enemy. Keep casting it until you have made an opponent kill another opponent 20 times, at which point, the trophy will unlock.