I was on level 17 forever, and by chance I decided to play this weekend and finally get it done. Imagine my surprise that I got an Anniversary portrait and the EXP boost. It did take me like 11 games to get to level 20 - I initially played Quick match - but I found I got more XP when playing Versus the AI [your team is all human though
], and the wait times were way shorter.
And please note, when you get the achievement, Graves goes into your bags. I was expecting an email or directly into the Pet Journal. I didn't check my bags... so I deleted the Cache, WTF and Interface with no luck [obviously]. So check your bags