I always find finishing off my opponent tricky in nine men's morris. If they have 3 pieces and can fly, how do I guarantee a mill to secure the win? What's the minimum number of pieces I can do this with?
You can't guarantee a win. In a 1996 paper called Solving Nine Men's Morris, Ralph Gasser describes how he used search techniques to determine that, given best play, the game of nine men's morris should end in a draw.
In figure 11 of that paper, he gives a graph of the percentage of won positions for the player to move, against the number of stones each has on the board. The highest percentage of won positions is 83% when the player to move has 3 stones against 3.
The data also reveals that a player with 3 stones left cannot force a win against an opponent with 6 or more stones left. Although many of those positions should still end in draws, rather than wins for the player with more stones.
A player is much more likely to be able to force a win when they have 7 or more stones left, than when they get down to 6. A player with 4 stones against 3 can only force a win if it is their go, and they can immediately make a mill.
All of this assumes two players who are playing completely optimally, which is not usually the case. In 1990 six games were played between the British champion Mike Sunley and a heuristics-based computer program. Two games were drawn and four were won by the computer.
Always keep in mind that one way to win a game is forcing your opponent into a position where they are unable to make a valid move.
When I'm up material wise I almost always opt for that win over reducing them down to three stones because I find it easier to calculate. In my experience with a 2-stone advantage it's reasonably easy to pull off.
There is a winning opening: Any corner of any quadron. If opponent does not play same quadron, you follow the corners of the quadron until you have a double mill opportunity. If opponent plays same quadron, proceed to next quadron with same strategy. If opponent plays same quadron all 3 times, there is almost always a way to connect existing pieces to a double mill opportunity, unless opponent knows that, in which case he can block.
You are now way above beginner level.
Nanana, you need just need make sure to have two open mill's at the time of cutting him down to three pieces, to force a win instantly. Of course to do this, you need a solid material advantage. However, there is an opening, that secures total dominance right from the start, provided you're the one that starts. Well, at least I have found/seen no effective defense against it yet.
(It's not the one mentioned above, but I don't have the language skill to put it down in just a few words.)