Anyways, I was able to unlock all bosses (Minus Kasumia Alpha I think) through Arcade. After getting Genra, I think it took me a couple more plays of Arcade till I unlocked Alpha 152. I would attempt Tag Mode if you don't want to go through the wait of playing Chronicles.
I haven't even played Tag Mode yet, but I got all 5 boss characters unlocked at the same time after an Arcade game. I'd finished the Chronicle mode and played a lot of the arcade, mostly just to unlock alternate appearances for lots of people.