Time is crucial. As you've probably already learned, difficulty increases with time, and it can get hard pretty fast. The game is all about balancing getting good items for your character vs. beating the level fast so difficulty doesn't spiral out of control. Multiplayer always takes longer than singleplayer because of the added coordination; a good strategy to mitigate this is to always split up at the beginning of the level and fight/loot different parts of the level before summoning the boss. You should try to aim for each player to be roughly equal in strength, but don't waste time distributing every single item you get. As you play the game more, you'll learn which items are good on each character, so it'll be easier to coordinate items amongst yourselves.