I've found one *somewhat* time-consuming trick to getting experience level 70: Online friends. Either you or your friends that play this game must be at a very high paragon level (I prefer your friends having that high paragon level) in order to do this. With you being behind the entire fray while your friend(s) tanking the mobs, elites and bosses, with you trying not to die at all, experience leveling should come much easier no matter what the difficulty setting is. The reason being is that whatever your level is, combined with the levels of your friends in the party, the game's settings matches those levels to match your skill. Almost essentially, the higher the difficulty level + the higher experience/paragon level your friends are = quick-and-easy power-leveling. I've done this myself with two of my friends on Torment V during Adventure mode, got to level 62-70 in under 20 minutes.
Once you get to level 70, BAM. *Cue trophy received notification*
S/N: I'm sure there's other -and better- guides out, this one is a rough draft of a guide I came up with.