Ghosts are found in later levels of arcade mode as well as ghost battle. Simply choose a character you are comfortable with and go in to ghost battle, defeating 30 of these characters. You'll known you've found one in arcade mode when you see a yellow name and win record on their side in the loading screen. If you want easier wins, simply choose lower ranks than you and you'll rank up slower meaning the foes will get better at a slower rate as well. Simply watch your win loss record to keep track of how many victories you have obtained.
It's also worth noting difficulty, round time and number of rounds are changeable for this mode as well. Naturally, if you want your matches to go by faster lower the difficulty, number of rounds and time per round and you'll have your achievement in no time.