The world bosses are not supposed to be beaten in one go, unless you have a lot of cards and upgraded abilities. They want you to upgrade and get stronger for each time the portal opens again. If you reach 25/25 for the boss, then you can only get the farm rewards after that. You can only get the farm rewards once each time the portal opens, so it is best to get it near the end when you can no longer win again fights this time around.