I don’t recommend worrying at all about this challenge. If you play regularly, and don’t try to avoid killing stuff, that will fill up naturally and eventually grey out on its own. Don’t waste the effort trying to get it done sooner.
I love what you’ve done with the spreadsheet. The data is not only helpful, but the layout makes it easy to find and understand. I’ve worked with enough spreadsheets to know this was well thought out and took a great deal of work. Thanks for putting it together and putting in the hard work. Great job!
S3:8-2 or 9-2 give about 17 enemies, so still better. 8-6 however is best for yellow chest, even without VF is a bit better than S1 stages.
The best stage overall for enemies is S2:4-3N during AR, about 18 enemies for 3 flags.