You will need to kill all the wedding attendees showing up on the overworld map. Killing the last one will unlock the trophy.
Start by setting up treasure box traps along your tunnel entrances. I usually pair rolling stone and treasure boxes for maximum defense and put 3-4 sets of them along the tunnel. You'll only worry about the gnome who can disable traps. Just pick up 2 orcs to dispatch him. Once everything is settled, prepare your army to intercept the nobles. First the first few nobles, I just use 4 orcs and 1 naga. However, upgrade them fast to the next level to unlock their secondary skills to fight other nobles who use force fields. Try to dig and follow gold around your dungeon. Some rooms have allies on them who you can add to your army for free.