He needs to (A) figure out why Shizuku would want to kill him – he doesn't know what Karin knows; and (
convince Shizuku not to.
Otherwise, he's going to end up like the Wicker Man.
Also, he might be able to change Shizuku's heart by getting her to be more open about her love of manga.
I do not think he's going to go the Karin or Soyogi routes. 1st girl wins. This isn't a case where one girl was introduced, and then another was a later love who supplanted her naturally. It's all been a very unnatural harem of sorts, and ripe for this sort of death by Cape Maiden plot. In a case like this, the hero doesn't do the easy thing and choose the girl who wouldn't sacrifice him; he'll know that Karin and Soyogi have already been changed, and he needs to find a way to save Shizuku's heart and personage too. Then, the manga might end with a Shizuku route, but more likely, it'll end open-endedly without him choosing between his three LOVES, even if Shizuku is the chosen Cape Maiden.
You don't go and tell someone, "don't choose x" and then have the character not choose x. It's schmuck bait, the Forbidden Fruit. Or rather, if this was Kami-sama's logic, then in video games, if you want to advance the plot, you always do either (A) what people tell you TO do, or (
what they tell you NOT TO do. If they're speaking in the negative, then you have to disobey them. There's no plot otherwise. You've got to purposefully spring the trap.