Just some tips if people are struggling doing this:
Sneed + Shredder: You can do this without being hit (unless your a silly loladin), the wodden path leading to locked door happens to not allow mobs to run sideways off the edge. So initially stand (when the room is cleared) on the wooden platform then wand / shoot / cast on the boss and when he gets near or when you get on low hp, move right a bit on the normal floor and bandage while he runs to the locked door and round back to you. This fight is easy anyway there should be little problem.
Smite: Kill the adds first. Then try and stay alive and damage Smite a bit while kiting him back near to the cannon used to open the door. When he stuns and runs back to his chest you have ample time to bandage before he gets back, stay by the cannon and wait till he comes back to you and do the same again.
Van Cleef: I'd recommend killing adds first, but you can do it with them. Pull him back to the side of the ship you just came from (approx above the wheel), do some hurty stuff on him till you're on fairly low hp (save some for the next bit) drop down onto the deck below, be careful not to fall onto the wheel and reset him or into the water as that just gets messy, bandage up and kite him back to the top into position to fall back down again for your next bandage.
At 19 I'm pretty sure every class can do DM, if you really wanna do it a full clear might take about an hour and probably more then that on your first clear.