I've found that if you complete each of the four heart events of a minor (i.e. the random people you meet that aren't from the anime) character, which is a tag duel tournament, the prize you get for winning is 80% of the time a card from the Assault Mode Archetype (can be the /Assault monsters, Assault Beast & the like, or the Synchro Monsters minus Stardust and Red Demon) Other than that, I've gotten Assault Mode monsters from the packs that the characters that I've finished (all four heart events) give me.