1. Open you Dedicated server's [profile_name].Arma3AlphaProfile configuration file.
2. modify or add these parameters:
difficulty="mercenary";
difficultyMP="mercenary";
3. Join the Dedicated server.
4. Login as admin.
5. Wtite "#missions" in chat.
6. Pick a mission and start it (do not choose the difficulty setting manually).
7. Check your difficulty setting in game. It will be all time Regular.