MySQL question - Data Cleanup ideas needed for Oracle guy doing mySQL

So, in Oracle when considering deletes and truncates, where the data to be removed is millions of rows and we only want to keep like 5% of that data (or some-such), I normally recommend saving off those records they want to keep into a temp table and truncate the original and re-insert them back.

In MySQL these tables may have an auto-increment field on the tables involved so I'm unsure if we'd be able to insert the data back without the auto-increment fields changing the data (or erroring completely).

So I'm looking for "good" options in mySQL to do this for you guys that do a lot of mySQL :)

Scenario:
Regular hash table with 28M rows. They want to keep say , 1M rows of the newest data (perhaps less/probably less) but use as an example.

I know we do a delete ... where <some condition> limit 1000 ; to incrementally delete and then I suspect we'd need to do an optimize on the table (another operation I'm unfamiliar with)

I've seen posts/topics recommending creating an index on the field used in the WHERE clause but I think that's a bad idea as it incurs addtiional index maintenance for a delete.

I've also seen posts/topics recommending creating a table with the keys you want to delete and doing a join from the main table to the driving table for the delete but I'm not convinced that's better than saving off the records and truncating the primary table and re-inserting.

So any suggestions/thoughts/ideas are very welcome!

Thanks,

Chris