The fastest way to level up is buy using /set, using this command will grant you a 4x experience boost and low tiered items to start off; your experience gain is boosted by the amount of players surrounding you, more people the better. Most people will use this to their advantage and teleport to the center of the map where the enemy spawn rate is higher.
Each character has their own statistics and these statistics have limits, each character has their own limit as well; the main purpose is to collect potions that permanently increase your stat for your character and to hit that statistic limit, when you are at maximum for your stat, it will change to a yellow colour, and you can later increase those stats even further after Ascension.
Try to kill as many gods/events as possible and follow players into Dimensional Rifts for legendary essence, sacred frags, onrane or gold.