Having him be a random world spawn would be fine, if there was another more organized way to fight him. When he spawns in the world he could be a public enemy, and dealing enough damage could cause him to retreat, and reward players who participated. But if you want to kill him for good, you have to pursue him to his lair and fight him with your LS.

The world spawn would have to be done right though, him flying around in the sky, and a decent amount of foreshadowing to his arrival, and perhaps fighting some NPCs etc. And of course, some sort of method of telling players that he's appeared. Him just appearing on top of a lv24 botanist and then wandering around in a field somewhere where no one knows he is, is no fun.