"possible to kill" doesn't mean that any reasonable player will be able to just pull it off. Like I said many of these fights were ALMOST impossible. They don't expect people to beat them, but it doesn't mean it couldn't be done.SE stated it was killable with only 18 75s at the time.
Long story example follows:
Pink Bean in MapleStory was almost unbeatable with 30 max level players within the 1 hour time limit (and its very hard to reach max level in that game) when it was added; it held a lot of uber drops that no one was able to obtain. Later on, they added new item mechanics in an update- basically you could enchant your gear to get percentage boosts to your stats, and after this system was released, people were able to become strong enough to kill it within the time limit. However, only a short while before this update was released, a group managed to beat it with a minute or so to spare. It was discovered that Pink Bean has this ultimate attack (that it only uses once I think), and the clock gets extended by 20 seconds times the number of people that get killed by it when he uses it- so it's possible to get extra time. At first people didn't notice this because for some reason the on-screen clock only updates to account for this if you die; and dying in a big boss fight ususally means you're out of the fight unless you had bishops with you to ressurect (and ressurect has a very long cooldown). Naturally this means buying ressurect items from the cash shop so you can keep fighting after the time limit is extended.
... Long story short, now a strong group can beat Pink Bean in about 15 minutes, and there's a new almost-impossible boss to take his place. Eventually, updates will make that boss easier too.
The point is, this happens in every game, and it's practically for the best to not even try to figure it out and just wait until it gets nerfed.