I want outside assistance by having an .htaccess rewrite.

I've got a site which had this permalink: /archive/%post_id

and transformed it for this: /%postname%/

I've attempted each and every permalink redirect and alter wordpress plugin - not one of them work with this.

They have 15,000 posts, adding one line 301 redirects within the .htaccess file is not likely to work.

Can someone assist me to give a rewrite which will redirect all cases of /archive/%post_id to /%postname%/ ?

any help appreciated!

I understand you stated you attempted all of the plug ins, but simply just in case, have you try that one:

http://www.shoutmeloud.com/how-to-use-deans-permalink-migration-plugin-for-wordpress.html

Appears like it dynamically produces 301s for you personally without Search engine optimization damage. Designed in 2006, working in '09 - you may have the ability to tweak the code to have it working the way you want.

If that's the 'quick' solution then your 'long' solution is always to write an application to research the postname by postid inside your mysql db after which change each publish URL inside your wordpress database using the postname value.