Unfortunately, you can't if you don't have the right number, i.e. 250. I killed Yielotax every day, and I know how it works, if you kill only 200 and someone will kill a boss Raging Mage, unfortunately, but everything will reset and you must kill 250 again. If you kill 250 pieces and you fail to kill the Raging Mage , the next day you have the option to kill and you don't have to repeat killing 250.
You need to kill 250 Yielothaxes in order to kill the Raging Mage and this won't get reset unless you loot the Raging Mage. This means if you kill 200 Yielothaxes and the Raging Mage spawns, you won't be able to kill the boss but your kill counter won't reset so you just will need 50 more Yielothaxes and wait another time for the boss to spawn. The Raging Mage will only spawn once per server save as the dimension to the Yielothax will close 5 minutes after the Raging Mage spawns, keep in mind the portal also opens up at a random time every day, so it's best to see if anyone knows if the boss spawned yet. You can tell when the boss spawns as there will be server log messages. Keep in mind your world needs to kill 2,000 Yielothaxes in a single day in order for the Raging Mage to spawn. The world counter gets reset every server save! I want to back up my answer with my source, a CM Response:
"Your individual counter will only reset if you loot the Raging Mage. The world quest counter will reset once the Mage died or after a certain time if your world does not kill enough yielothax...The world counter gets reset every server save and the world quest starts again randomly."
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