I’m trying again with adding the chapter index in the description box. Please let me know how it looks.

I don’t see how Trench would be of any use against a flying creature. Since the giant wasps are the creatures we’re most likely to deal with, that makes it less useful.
 
As for Shield, I’m assuming casting a shield would mean having to stay in one place which, if you’re trying to get away from something, would mean being trapped and how long would Emerald even be able to maintain the shield while it’s being pecked away by giants wasps? Leave a spell like that to Joyride which she already has and can cast it pretty well.
 
Even if the shield can move with you while you run, that’s still not going to shake whatever’s chasing you. I say a good defense is a good offense. That’s why my vote goes to Whirlwind, especially if it’s capable of affecting more than one giant wasp at a time.