The lower floors you should do small dungeons. Best to go in a team, so the difficulty is 5:5.
The higher floors you should do large dungeons, but I don't recommend this until around floors 25+. Again, team of 5.
Best to have nobody that is too high levelled in the team, cause the monsters will be high levelled as well, making the dungeon harder. The more deaths you have, the less experience you get. Each death takes away around 5 - 10% of your final experience (never go over 3 deaths).
When a player dies, they usually rage quit, which is the most annoying thing when doing a large dungeon, as it takes around an hour to complete and requires a full team to do it well. So try and find a reliable team, that won't quit.
Never do the same floor twice. You won't get prestige experience. When you have done all the floors, rest your prestige, and you will get a nice prestige bonus (which will get you a bigger final experience count). Don't reset if you haven't completed all the floors that you can because the prestige bonus will be lowered, and you will find that your experience will be much lower.
Hope this helps
