To earn this achievement, "Dungeon Fighters unite!", you need to be in a party of three.
One of you has to be a fighter, the second has to be a gunner, and the third has to be a slayer. For example, you are the slayer, your second buddy is the fighter, and the third buddy is a slayer. It doesn't matter what order as long as all three classes are seperate from each other.
To make this achievement go faster, just quicky go through Lorien on Normal, which is the easiest dungeon in the entire game. You don't have to complete all the areas, just the dungeon itself. So you can just find the shortest path to the boss and kill him.
I doubt that this can be offline, so this has to be done online. Also, all three characters must survive the dungeon together for the achievement to unlock it.
I didn't do it this way, however. I was attempting to find a game in Poison Thunderland, on King. I found a game and dropped in. My class was a Level 20 slayer, so I came into the room with a Level 20 Fighter and a Level 20 Slayer, who were taking on Fenrir. I just happened to drop in right as they beat him. Once they cleared out the boss, the achievement unlocked.
Those are just two methods, but again, to make it easier.
-Find two friends (or strangers) and make sure each of them are a different class. One fighter, one slayer, one gunner.
-When you have done that, just go to the easiest dungeon in the game, which is Lorien on Normal.
-It should take about two to five minutes to take the quickest path and reach the boss.
-Once you are in the boss area, just rush up to the boss and pound away at him until he's dead. All three must survive in the end.
-If all three members survived the boss and the dungeon together, the achievement unlocks forever!
Have fun and good luck!