So, there seems to be some confusion(Myself including until recently). What you have to do is NOT get 20EXP overall. Bungie updated it who knows how long ago, and added in the playlist EXP thingy, so if you have 12EXP on lonewolfs, you should get it. If you have 12 EXP on Team Slayer, you should get it. If you have 12 EXP overall, then you will NOT get the achievement. Any questions, ask me.