How does one get to lvl 100 in one day?

So a lot of people say that they can get to level 100 in a day but how is it actually done? In EMS I managed to get to 120 in a day with all sorts of exp boosts and events but how does one get to lvl 100 without 2x exp cards? I tried just playing but managed to get my character to like lvl 70 in 2 days. Following training guides has also managed to get me no results. Any suggestions would be great since I plan to get 3 characters to lvl 120 for their link skills fairly soon...