Here's an efficient way to delete your texts:
Pick a starting point (from top to bottom). If you know what you don't want...say the next 10 threads, then all you do is just start swiping like you would in the email app. It will ask you "delete" or "cancel" but ignore that and go to the next thread and swipe. As you swipe each one by one, it'll delete it without you having to click "delete". That kept me from deleting my messages cause it was so cumbersome. But now I just swipe like crazy and I can get rid of 10 threads in about 5 seconds.