It's probably easier to use the website interface to do this.
If it's an iCloud account you can use the icloud.com site from a computer browser. You won't find the exact tool you want, but you can open a folder, select an email by clicking on it then scroll down and shift-click on another to select everything in between then delete those. Slow, but better than what you can do on the iPad.
I don't know about other email providers.