You'll get this achievement at 99,999 xp, which will take quite a long time. With the most efficient grinding methods, you'll still only be able to rack up about 6000xp per hour.
What I did was play as a golden character as soon as you unlock one, as you get bonus exp for playing and winning a game with a handicapped golden character. Winning a game flawlessly as a golden character will give you from 120-132xp or so on average, and on fast maps you can win about every minute or so.
You can do this in custom multiplayer games against easy bots (who could be handicapped as well), but that could get boring pretty quickly, but any difficulty/handicap gives the same xp so do what is the least painful :P