To earn this achievement, "Ultimate Dungeon Fighter", you must have any of your classes reach the maximum level, which is 20.
I finally earned this achievement at the Shadow Thunderlands on Normal, after I killed the last Forest Zombie in one area. If you want to earn this achievement faster, you're better off playing solo, in my opinion, as you'll earn exp alone for every kill you create. It also helps to create high combos so that you can earn a better ranking after finishing off the boss of that dungeon and earn more experience. You can earn the achievement with friends too, if you'd like, but if you want to earn this achievement at a faster rate, then go through the game as normally on your own.
Keep in mind that this achievement can be time-consuming to work on, but the moment you reach level 20, the game will pause for a second, then the game will continue with you leveling-up to 20 and your achievement should unlock right then and there!
Happy hunting, have fun, and good luck!