So - it's pretty much similar to beating the post game boss - with a low level speed run character party.
I do agree that the visit to the demon Lord's castle now took on a whole new meaning - Wasabi at the time was just sizing her up as a potential future rival toy - but if this is to be believed, then the Demon Lord was sizing her (and Wasabi) up so that he could die.
It's funny though - for the ones that created both demons and angels that they gave Aki such a disproportionately sized Angel. Granted, it's because of Angel Aki that Aki can even exceed limits, something that the brother / sister wanted.
But yeah - if you think about it, the only way Wasabi could possibly die is if someone can break the speed limit. The only way the brother / sister can die is if someone can break the status limits - and from what we can tell, the only person in that game world able to do so is Aki.