The Guardian, or as he is known in-game as B-DSS Knight, is one of two bosses you can fight in the Factory. As you progress through the level, you will see rooms coloured orange; this is the area that the Guardian is found in. Natural exploration (or fleeing all the rooms) will lead to you finding the boss room for this achievement - which is marked by a special symbol on the map - and you will be unable to leave the room once you enter, so make sure that you are prepared. The Guardian is weak to lightning and will be stunned for a while if you deal enough lightning damage and so it is advised that you have a lightning weapon upgraded if possible.
The boss itself isn't particularly difficult: he only has a few attacks which can be easily avoided. Firstly, he can shoot a number of lasers at you (a maximum of four at once for me) which can be dodged by simply running in a circle. He can perform multiple slashes in a combo which are not very effective if you just stand a good distance away from him. He can also charge at you, when he has low health and subtly changes his style, which can be scary but just run or dodge away from him until he attacks the air.
Generally, you will want to wait until he has done one of his attacks and then go in for your own and come out quickly. You can usually get around two hits on him after each attack of his before you should get away so he doesn't hit you. Make sure to tighten your circle around him when he shoots lasers as this is a good time to counter-attack and keep your distance when he is doing his melee attacks.
After defeating him, you will receive this achievement and spawn in the City area of the game.