How to remove bad Pandalized content properly?

We have 10,000 of bad pages, which panda penalized us for that.

If we delete them we will get 404 error, and after that we could again get penality from G algo.

How can i delete them to follow google rules and avoid penalities?

If we make redirect of 10k pages with 301 redirect to index, can 10k old pages be treated as duplicate?

Only you know 100% whether the content is duplicate as you are the one who is responsible for content!

Hence you need to carry out a content audit and start deleting the pages that are BLATANT duplicate content.

i am asking where technically i should redirect 404 pages?