In an optimal solution for both players, Black needs 6 moves to defeat his opponent. There is several ways to obtain this result. I would suggest this one because I find it fine :
f6e6 / a5a4
c6b5 / b6c7
d6d7 / c7b6
c5d5c6 / b6a5
c4c5 / f5e4
c5b5 (winning in 6 moves, 6-0).
Special thanks to Lolo for his help in the solving of the problem.