The first chance you'll be able to find a Plains Guardian (Nadaig Magamen) is in Glendgnámvár, "The Shore of the Tall Bones". Kind of in the middle of the map shown here with the red marker:
You will also encounter a Plains Guardian during the side quest "The Origins of Theleme - In the Footstepts of Saint Matheus" in Vedvilvie near the Cousoneigad Camp. (Unconfirmed if this one pops the achievement.)
Make sure to save your game before starting the fight.
The Plains Guardian is massive in size, has a ton of health and armor, and wields a spear that has a pretty good reach. He will also occasionally throw it at you. Make sure to pay attention and be ready to dodge at a moments notice. His attacks do hit very hard, but are easily dodged. You can also parry the Guardian's attacks, but I was using magic so tried to keep my distance as much as possible.
If you have a Firearm I suggest you spam fire it at the Guardian to get it's armor depleted as soon as possible so your companions can help you DPS. I'm a mage and had Vasco and Siora with me. The Guardian mainly focused on me though, so you'll need to be quick with dodges while you DPS. Remember to use your Fury attacks with
as often as you can. Make sure to get some distance beforehand and watch out for the spear throw. Once you get the armor down it's just an endurance race of dodging, letting your companions deal damage and doing DPS as safely as you can. I did this at level 12 and only died once because I got myself stuck against a rock and couldn't doge out of the way. As soon as you kill the Guardian the achievement will pop. The Guardian also drops it's bones as loot which can be sold to a merchant for 150 gold.
If you find yourself still having some trouble you could try changing the game's difficulty to Easy in the settings.