Level Up Quickly
Hidden around the map, you'll find XP boards. There are a total of 100 of them. The most common ones are worth 1,000 XP. You'll find 50 of them, if you search well enough. Less common are the boards worth 5,000 XP (there are 35 of those). Finally, there are 15 of the rarest boards, which award a whopping 20,000 XP apiece. Finding those should allow you to level up very quickly indeed.