You can locate the Swamp Guardian (Nadaig Vedemen) in Magasvár. "The Vale of the Great Battle" in the Hot Springs area just north of the Smouldering River Camp, shown here with the red marker:
Make sure to save before starting the fight.
The Swamp Guardian is probably the easiest of all Guardians so far, but is still a challenge. The Swamp Guardian has a lot of health with just a little bit of armor. Knocking the armor out should be the first priority so your followers can help you with the DPS. The Guardian does have tentacles with a little bit of reach, but most of their attacks are slow. You'll have to watch out for the Guardians gas clouds which come in two forms and can drain your health if you're not prepared.
I was playing as a mage so I was able to keep my distance and easily dodge attacks, but the Swamp Guardian is able to emit two types of poisonous gas clouds that can eat your health if you're not careful enough. One gas cloud will simply engulf the surrounding area around the Guardian. If you're a melee type character, I suggest having high poison resistance or have some Antidotes bound to an easy accessible button to avoid taking a ton of damage. The second gas cloud will actually follow you for a short time. So if you can, sprint in a circle around the Guardian until the gas cloud goes away, or simply take another Antidote to ward off the effects. Chip away at the Guardians health and once you kill it the achievement will pop. Looting the Guardian's corpse should give you 3 Obsidian which is very useful in crafting. There's also a dead Guard underneath where the Guardian was standing. Looting the Guard should give you some great loot. I'm not sure if it's randomized, but I got two unique items and a key for a chest from the corpse.
If you find yourself still having some trouble you could try changing the game's difficulty to Easy in the settings.