I have noted that in some of my sources the extreme/challenger puzzles are unsolvable by logic and you have to "pick" one of two routes and see which one is a "bust" to actually solve the puzzle.
There are actually some very complicated schemes to get through the evil/insane level puzzles, but I only know about those, not how to use them.
What platform are people playing Sudoko on? I am using sudoku.com, mostly because I like their interface for the most part.
Gumby, I find that (on sudoku.com, anyway) the "Evil" level is often quite a bit easier than the "Expert" level. What I usually do is repeatedly click on "Expert" until I get a game that less than say, 45%, of players are able to finish. It seems to me that I can often complete "Evil" using only elementary techniques, but I have to pull out the bigger guns for "Expert."
But 7 or 8 minutes is impressive! I just cranked through an Evil level in 13 minutes; it was not difficult, but it felt like a real rush.
I don't know about that site, but the most difficult puzzles handed out on a recent cruise I was on certainly couldn't be solved with elementary techniques!
This cruise that I just got off was my first foray into sudoku. It was exclusively paper. My wife had an app on her phone and it would tell you when you blew it! Totally cheating! I remembered one rule someone told me: only pencil in candidates if there are two, and I could solve all except "insane" level.
When I got back home, I found a site where you could type in a puzzle and it tells you what techniques are required to solve it. Some of those techniques are completely unintuitive... I'd never legit solve one of those. Sure, I could stumble into a solution, but only at the risk of going down an incorrect path.
So I think I'm done with sudoku. That's a pattern I've noticed in myself (getting back on topic): once I get a handle on something, know the ins and outs of it, even if I haven't mastered it, I move on to something else. And I think that's actually better than doing the same thing "forever".
My puzzle today is trying to parse a PDF. More accurately, see why my earlier design is crashing. If anyone wants to help me with this puzzle and knows how to code in Java, let me know