I get the feeling, that the Dark Hunter is significantly weaker than other classes for the front row (Landsknecht or Protector). I have a party of around level 25 and I'm in the Dungeon on B7 and my Dark Hunter dies more often than the others. Is this class not comparable to the other meatshields, or do I play/build him wrong? On the build-department, I concentrated on the attack-department (but did the same with the Landsknecht) so I increased attack up, whips and some whip-skills. Is that a wrong build?
Unfortunately, your suspicions are correct: the Dark Hunter is considerably less survivable than the meatshields. The trick, though, is that they're not a meatshield. The Dark Hunter's speciality is inflicting status effects like binds, and (at low levels) supplementing your damage dealers. If you were a little higher in the Labyrinth I'd suggest swapping them out, but those binds are going to come in handy in the stratum you're in, and as the Dark Hunter levels, their damage dealing becomes increasingly worthwhile.
Dark Hunters are probably best used as a supplement to your main damage dealers; I'd start building towards the binds. Dark Hunters can only damage from the front row and tend to be high on the move order, so you can ensure their survivability by correctly binding whatever enemy you're facing.
Remember, also, that Etrian Odyssey is a game where party composition is very important. Don't expect to stick with the party you have for the entire game.