For all achievements regarding "Reach Level xx with yy" I suggest playing through mission 2-1 Crash Site. You can finish the mission in under two minutes and it gives you between 15.000 (Easy) and 19.000 XP (Hard).
Even with a Level 1 character it is absolutely no problem to play this mission on Hard difficulty.
Always skip the dialogues with
and head to the mission markers, ignoring the enemies.