If you want to delete large number of posts, pages, attachments, users from WordPress in scenarios like you are creating lot of auto generated posts or you have migrated your current website to some other domain etc, in such cases we have been using WordPress Free Plugin called “Bulk Delete“
Note: This posts recommends “Bulk Delete” plugin since we have been using for one of our website to successfully delete large number of posts, if you came across any other plugin, you can suggest the same in comments.
Install the Plugin from “Plugins->Add New” and Search for “Bulk Delete”
Once you installed this plugin, from Admin sidebar, “Bulk WP” , click on “Bulk Delete Posts” [If you want to Pages, Users.. select related option]
Following image shows, how you can delete all the posts permanantly by skipping trash.