The Medium A.I. is pretty unforgiving. You want to flip as few pieces as possible early on. The goal is to give your opponent as few options as possible. Try to avoid giving your opponent the corners, or the 3 adjacent tiles to the corners. Taking the corner-adjacent tiles will lead to your opponent taking the corners, making life difficult for you.
This medium A.I. could make a Monk punch his mother in the face. Never in my life have I played a more infuriating A.I. I have 12.9 hours in this game and 4 wins and 39 losses against the Medium A.I. Honestly, there's not much I can say other than keep playing. I know that's not super helpful, but this game's A.I. crushed every other Othello A.I. I threw at it, with one, extremely aggressive exception. (The Othello set to maximum spanks this game's hardest difficulties every time.)
Keep playing. Keep playing. Keep playing! The only way to get better is to practice. Due to how buggy and dead the online is, your primary opponent is going to be these A.I., so just keep at it and you'll grow as a player and get it eventually.