add a floor clearing bonus! unless those coins were the floor clearing bonus and i didnt realize. also if you end up extending the runs, make the demon become 0 whenever the whole board in its entirety except the demon is cleared. it would be satisfying and makes sense.
if you want him to remain a "undefeatable foe" for some lore reason you could make the space be empty when you click him and you get no xp to show he escaped.