"some people" are wrong. Not all opponents will be unlockable. It's like expecting Tour Guide or Imairuka to be a playable character. They're obviously meant to be NPC-only. I wouldn't be shocked if we got, say, Strings eventually, but generic Rare Hunter and Mind-Controlled Joey are unlikely.
You use Alexis, then choose a lvl10 LD from the gate with weak monsters(tea for example) and then you use a very strong beater deck(1800+ monsters). When you go first, then you set a monster with high def(so the AI attack into it for big damage). Else just attack. Quick Victory is achieved, when you win within 4 turns.