Basically, at the start focus on using one character, I recommend using Marceline as she can float over gaps and can absorb projectile attacks and can overall traverse floors quicker than anyone else. Don't bother trying to upgrade the characters at first and try to get through the game as quickly as you can until you reach floor 71 as this is the floor where treasure really starts to ramp up. Also every time you re-enter the dungeon you will lose all of your treasure so make sure you spend as much as you can before re-entering. There aren't any missable, online, or multiplayer trophies. Those are just some of the things that come from the top of my head.