The Axii sign can be used to turn an enemy into an Ally for a brief period of time once you've unlocked the second row of abilities for the magic spells (The skill is called Puppet).
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.