To reach Niob difficulty and get this achievement, you have to do a playthrough of the Silver, Gold, and Platin campaigns. Once you've done that, simply start up a Niob game with your character and the achievement will unlock. You don't have to beat the Niob campaign for this achievement. The difficulties are not stackable, so you have to do separate playthroughs for each difficulty.
Some advice:
1) Start on Silver. You can start on Bronze and do this, but you're adding an unnecessary playthrough, and more importantly you're risking getting to Silver with a character that is weak because Bronze was too easy to point out weaknesses in your build.
2) Use someone else's build. This is the most important piece of advice I can give. Unless you're a seasoned Sacred 2 pro, this game is NOT noob-friendly and the game WILL punish you if you don't know how to build a well-balanced character. Trust me, there is a lot of information that the instructions/in-game tips don't tell you, and many people have put hours into this game only to find themselves suddenly dying at a higher level because of a weak build. So to save yourself some grief and help you learn the ropes, find a good build and stick to it. Personally, I'm using lordraven's Seraphim build and it's working out beautifully (
http://www.gamefaqs.com/boards/genmessage.php?board=938603&a...). For more information on understanding how a build works, Sacred 2 Wiki is your friend (
http://www.sacredwiki.org/index.php5/Sacred_2:Fallen_Angel)
3) Use this character to get the longer/harder achievements. On your way to Niob, you'll have plenty of time to get some of the bigger achievements like killing 10,000 enemies, doing 200 quests, exploring 50% of the map, and so on. Some achievements like character-specific achievements, Scrooge (don't buy anything for a whole campaign), and Survivalist are best saved for shorter playthroughs with different characters on Bronze, so don't worry about those while on your way to Niob. Xbox360achievements.org has a great road map for thinking about how to organize your campaigns (
http://www.xbox360achievements.org/game/sacred-2-fallen-ange...#).