Mass deleting email from an iPhone/iPad?
The fastest way to delete groups of messages in the iOS mail app is to hit the edit button in the mailbox view, and tap the circles to select the messages you want, the delete/archive them.

If that's still too slow for the quantity of mail you have, the best option is likely to use the web interface for your provider (i.e. Gmail, etc.), which will likely allow you to do a select all.