I used Min-max method for only one depth.
The first node has 50 branches. Second nodes have 25 branches each.
I pick up the top 10 largest cells which is not mine.
The opponent picks up the top 10 largest cells which is mine.
For each point, imagine what happens if I (or the opponent) choose that point. Calculate the gain of the total area (score).
"Choose the point which maximize (my gain - next opponent's best gain)."