You need to have high level monsters on a mono-color team, and you need to be taking advantage of EVERYBODY's skills.
For my mono-dark team, I lead with my Lucifer, who I've evolved already. He gives me 2 x HP & RCV for all of my dudes. I've also buddied a bunch of other Lucifer users, so that way I get 4 x HP & RCV total. I'm usually going into battle with over 40k hp, and I heal a lot when necessary. I run a Keeper of Rainbow for his massive HP total, and his skill halves damage for 3 turns. Then, I also have Hattori Hanzo, who has a very unique skill- he turns fire orbs into dark orbs, and a Chaos Dragon Knight, who can turn light orbs into dark orbs. They make sure I'm usually attacking, and if I need to clear orbs to get more to drop, I clear water and wood orbs, since I can't change them and they have no usefulness to me. Finally, I have a Lilith that I use because her skill poisons enemies, making it easier to grind them down.
The whole point of using this team is to take advantage of the two Lucifers skills- in exchange for setting my hp to 1, I can do a massive critical strike to a single enemy for about 300k of damage. 2 of those will take out a lot of bosses, and the rest of my set up- really high hp and rcv, ability to defend against dangerous attacks, poison enemies to gradually kill them, and change orbs so I'm continually doing damage- sets me up for a grinding playstyle where I kill a boss enemy all at once.
tl;dr- know your monster's skills. To get ideas for tough dungeons, look at the dungeon on pdx and read up on the suggested teams. That will give you ideas.