Instructions
Get three friends to play co-op with you. You do not all need to be the same level. In fact, if you are hoping to face enemies that are more challenging than yourself, you want some of the players to be a higher level than you. Ideally, at least one player should be Level 50 or higher. Any level over 49 will increase the enemy's base level.
Have the highest level player host a new game. The other players may join after he has entered the world.
Seek out some monsters. Not only will the monsters be more challenging, but they will also scale to the level of the highest level player.