5. Complete As Many Challenges As Possible
Every game mode, gun, attachment, perk, grenade, exo ability and scorestreak has a list of challenges attached to it. Upon completing each challenge you're rewarded with bonus of anywhere between 500 and 10,000 XP depending on its difficulty.
Top Tip: Variety is the spice of life. Change your weapons and attachments more often to complete more challenges and level up quicker.