I have one suggestion.Hold the victory announcement for 24 hours after he leaves the coaliton.If he is going to win solo he should be able to survive 24 hours alone.The 24 hour leaving period is not enough because you cannot take any action when he is in coalition and if he is smart he will leave the coalition at 23:55 so that when he leaves his coalition, his former coalition members have only 5 minutes to attack him and after 5 minutes the new day start and he win.
I am the best player of this game that was and ever will be