This is my first guide so go easy on me.
Ancient magic is the act of pushing both your bumpers (LB and RB) at the same time to do a special attack.
There are a few things I would recommend doing off the bat for this.
First I would highly recommend having a good portion of the map unlocked this will allow us to fast travel around quickly to find Bandit camps and Dens,
My second bit of advice is to have a few of the Ancient Magic spots done as this will boost our charge meter to hold more charges, meaning we don't have to keep grinding to gaining charge. Speaking of which I discovered a very easy way to do this, if you find an Inferius Den you can farm them for charges, one of the fun things about Inferius is that they do not take damage unless you set them on ablaze, so turn down your difficulty to easy or story, to allow for more time to react to counters/dodges and then hit them with basic spells to build up the combo meter and charges at the same time (this is also a really easy way to get the 100 combo Achievement out the way).
Now for the list of enemies, it can look quite daunting to begin with as the game suggests that there are 67 enemies in it however we can use common sense to discount almost half these straight away, as a chunk of these are boss foes or story enemies that we can only fight once or in specific story missions, we can assume these aren't needed for the achievement, that leaves us with just 37.
Human enemies are split in to two categories, Ashwinder and Poacher these can be found in Bandit camps.
Duelist, Scout, Assassin, Ranger, Soldier, Executioner.
Duelist, Tracker, Animagus, Ranger, Stalker, Executioner.
Loyalist are the goblin enemies of the game and can also be found in Bandit camps.
Assassin, Commander, Sentinal, Warrior, Ranger.
Trolls are interesting as they can be found in both Bandit camps and Dens, I believe the trolls wearing types of armour are in camps only.
Armoured, Fighter, Fortified these are camp ones.
Forest, Mountain and River are Den ones
(The Trolls are potentially also grouped into wild and tamed ones, meaning you only need 1 from the Dens and one from Camps)
Dugbog are in Dens.
Stoneback, Great spined, Cotton Grass
Spider there are two different types of Spider enemies in the game, Venomous (V) and Thornback (TB) along with the Acromantula which is it's own thing they can all be found in Dens.
Acromantula, V Scurriour, V Ambusher, V Shooter, V Matriarch, TB Scurriour, TB Ambusher, TB Shooter, TB Matriarch.
That leaves us with just the last two which can be found in their respective Dens they have no sub enemy categories so don't worry about that just go find and use Ancient magic on a single one.
Dark mongrels and Inferius.
edit; several of commenters have also informed that a normal mongrel is also needed these can be found hanging around camps with poachers.