Another quick way is Wolfpack if you don't have enough friends with the game to boost. I levelled up in 3.5 hours according to my stats. Use focus on kills, disguise, hide etc. and you'll rack up the bonus points for challenges. As a rough guide, I'd killed my target on the ground 300 times by that point so was getting 2000-2500 for completing levelled up challenges on a regular basis.