My idea for Modernist achievement is to keep rematching Shadow X (it is much easier and faster than final boss). He is level 9 and if you are good enough you should beat with him with almost not loosing any health (constant powers attacks) and each fight gives you 1400 exp.
After you are finished with the story mode you can choose continue and rematch. Then just select Shadow X. I played with Volt (not X) and Soothsayer X but anyone you feel comfortable with should be ok (except the ones that have shield powerups). Use your first, stunning powerup, then change, use your first stunnig powerup and then any other to have Ultra Chain. Then use powerups that are left. While your powerups regenerate just keep making close combat. Don't worry if you cannot make the last hit, you don't have to, sometimes loosing is winning ;]. Do not care about dodging any attacks, it is you that shall be in constant attack; good timing and knowing you attacks are of the essence. After a victory you can choose retry so you don't have to choose players. Each 10 000 exp shall take no more than 12 minutes. So it takes about 3-4 hours :].