Killing Chilltara 1000 times means getting the right dungeon 1250 times which means remaking the game 2500 times. My reading’s fine, so’s my comprehension, and I notice when you move the goalposts between “1000 kills” and “1000 runs”.
It’s not a 5% chance per game though, it’s 2%.
If I had already provided an answer to the question, over four hours ago, would that be a problem for your narrative that I’m avoiding it? Oh, wait…
Chilltara may well have a 5% drop rate of the item and, indeed, that’s the value I’ve been using, but you’re ignoring the fact that the right dungeon is only on the map 50% of the time, and the mob only spawns around 80% of the time the dungeon is there. So, it’s 5% of 80% of 50%, which is 2%.