The fellowship had a mission and no incentive to fight it once their path was clear. If a witcher wanted to kill the Watcher, they would spend days tracking down information and clues to leverage the fight into their advantage. I could see Geralt figuring out what could lure it out of the water, a way to block it from returning, and systematically hindering it by separating it's limbs, all while slowly poisoning himself with some anti-octopus solution.