This is a pretty big grind. You will likely have to play through the entire story at least 3 times, plus a little Mapwork dungeon running. So don't expect this achievement to pop until NG++, late into the story. Sadly there's no real way to farm for easy XP, you'll just have to play through the story and hit up Mapworks if you get bored with that. Always look for maps that offer 5% more XP and try to find gear that adds extra XP, even if it's just 1%, every little bit helps.
Good luck, and happy grinding.