I just unlocked the England boss, Merlin. He shows up in my fight list, but when I try to attack him I get the following message and it redirects me to a boss I already beat (clown): "You cannot attack a boss that is not currently in a fight." I don't get it. I haven't even begun to fight yet, just clicked on the challenge button then it took me to the fight page, but I get that message when I try to click on the attack buttons.
If you are the only one fighting a boss & you are getting this message you are clicking on the attack button too fast. The browser is trying to catch up with each action but can't keep up with the mouse clicks in real time.