The problem with this story is I want the bad guys to win. Reimu and Yukari aren't any fun, because they're freaking invincible.
Although I do believe that last word you just had Reimu cast would literally kill herself.
Reimu: Pass out.
Now, of those four, Yuuka is the one that doesn't need the spell card rules at all. She is literally a force of nature, and it is doubtful whether or not she can die, so she could probably take even an unlimited fantasy heaven, straight to the face, without spell card rules right up until Reimu passes out. But she's still stuck in that pocket dimension.
Yuuka: Break the eyes
Still, there's a good chance that won't work. What we really need is a way for Yasora to get at Yukari, Reimu's just the small fry. The problem with that is that on top of her undefined hax, Yukari never has to participate in a battle unless she wants to because of portals alone. So let's attack something Yukari will defend. Now, not even Yukari can make the Gensokyo border on her own (in canon), and that's important to her.
Yasora - Break the Hakurei border (with nuclear fusion)