Since the boss was a humanoid slime, it will be far easier for the three teenagers to beat him. But even though, it had limbs that they could use to attack the boss, it will still be hard. The slime was still a boss after all, and a boss couldn't be underestimated. Otherwise, one could only meet its doom.
So even though Eden knew that this boss would be easier than the last two, he still tried to be prepared for the worst. Only like this could he protect himself and his friends at the same time.
Also, usually, slimes had an element or an attribute that were their own like water slimes that mastered the water element, or the holy slime that mastered the holy attribute. So looking at this boss, the only attribute Eden could come up with was either an anatomy attribute or a creation attribute. Even though it sounded strange, it was still a possibility. And it was better to expect the worst to not be taken aback.