I was just about to show off my level 67 highscore. How did you level up so quick?
I took the Cursed XP perk every chance I got (Level up 20% faster, but all enemies have 20% more health).
I've also discovered a minor exploit. It seems that something funny is going on with how gold drop is calculated. It looks like the amount of gold is calculated by X amount per car when a shot kills the killing blow. Normally this works fine, but the blunderbuss can deliver several killing blows effectively at once causing a vehicle to drop its reward gold multiple times from a single fire of the blunderbuss. I'm not 100% sure that's what's happening, but it seems to be the case. You have to play somewhat risky to take advantage of this exploit by letting the incoming vehicle get super close.
The main thing that was letting me level up so fast was the Cursed XP though.