Update 2020-07-18: Added some faster variants and removed the old variant C (https://imgur.com/P9Grt21)

In Nine Men's Morris, one can win by either milling the opponent down to two men, or by blocking the opponent so they have no moves. Obviously, the latter one is faster, and this strat is designed to reliably get a win with at least four opponent pieces remaining. I doubt this particular strat is optimal for the IL though, since you do give an early mill to the opponent. This strat works decently well against any difficulty, although Normal ai is probably the fastest.

The start is pretty straightforward: On turn 6, replace the marble that just got milled. You may need to undo a few moves, but you're pretty likely to not need too many. Do note that the order the ai plays in doesn't matter too much, and there are obviously rotations and reflections possible; additionally, there is another game-theoretic symmetry by interchanging the inner and outer rings. These considerations can save a bit of undos if you have sufficiently decent spatial awareness, although they are not that important in the greater scheme of things.

Now there's 10 different moves for the ai and all of them can occur. We can distinguish four distinct variants: Some spaces are unmarked; undo if the ai plays there. Otherwise you can follow the branches as marked. In each variant, your last placement is marked with a ◆ in the corresponding image. The blue arrows show your mill setup in the movement phase. Note that we just need to set up at most one mill, since the game should end before the opponent reaches the flying phase.

Variant A (top-right outer) I know it looks like you're just gonna get blocked at bottom-middle-inner, but the ai normally considers the more indirect threat on left-inner more important. Just undo if you do get blocked.

There's also an IL meme hiding in here: if the ai plays top-left inner, you can just move ◆ up for an instant win. Don't undo for this in full game runs; it is too unlikely for that to happen.

Variant B (bottom-left/bottom-middle outer) Not much to say here, except that picking the correct marble to walk with is important.

Variant C (top-left/bottom-right outer) The picture shows the strat when the ai plays top-left; the strat for bottom-right is essentially a mirror (play top-left yourself and move anticlockwise)

Variant D (top-left/bottom-left inner) There's a marked path; you're always moving in that direction. First move your ▲. Now, if the ai moves its ▲, you move your ▲, and if the ai moves its ◆, you move your ◆. Repeat until you win. Don't question how it works, it just does™.