In the email account you are using - tap “edit” up above the list of emails. That brings up a checkbox. Simply check off every email you want to delete, then just tap “trash” down at the bottom. I just deleted 71 emails that way.
I know about that way, it just take ages to delete hundreds since you can't do a “Select Many” like a PC - it's a major defect in their mail system.
Even 71 clicks to delete a group of emails is a pain in the b**t and poor software design....:^)