Levelling you first character is relatively simple – explore the different zones killing enemies, complete the main story quest, complete all side story quests and complete priority quests to level up your first character and weapon.
Levelling additional characters and weapons is the bigger challenge especially if you try to level solo, it’s a lengthy task.
For additional characters one of the best places to gain experience is by doing the Ryv’n The Devourer on Sphere 4, this is due to the fact that it’s not super difficult to kill and you have many adds in here so you’ll gain additional exp for killing those. In a 3-4 min clear you can gain around 4-6k experience. When the exp gains are fixed this should go up to 12-18k exp per clear.
It’s far easier to try and group up with other players looking to gain exp. If you can put together a group of 3 people, have 1 person levelling a character but using a level 30 weapon combined with 2 other people using level 30 characters and levelling up low level weapons you should have no problem burning through Ryv’n quickly and everyone benefits from the exp gains.